1938: Geology

Estimates based on membership in American geological societies and available data from other continents indicate that more than 10,000 persons in different parts of the world are actively engaged in geologic work as a profession.

Many additional persons contribute directly or indirectly to advancement of geologic knowledge. The range of studies being carried on is certainly much wider than in any other field of science, for geology embraces application of all sciences to the enlargement of knowledge about the earth. Increasing specialization in the diversified branches of geologic research and the voluminous publications in many of these branches is unfortunately responsible for inability even among professional geologists to keep closely in touch with work outside of a particular field. Mineralogists, crystallographers, petrographers, vertebrate paleontologists, palcobotanists, micropaleontologists, seismologists, oceanographers, geomorphologists, and economic geologists of many special sorts — to mention only part of the list — devote most of their time to one division of geologic science, and divergence rather than convergence is the inevitable present trend. It follows that in the limits of a short article, such as this summary of geology in 1938, notice of only a small number of representative subjects can be given, but it is necessary to avoid condensation to a point of rather meaningless enumeration.

Oceanography.

Greater attention than ever before is being given by geologists to problems of the floor of oceanic areas, especially the more easily reached areas of shallower depths adjacent to continental borders. Important aids in this study come from development of new instruments and techniques, as well as from interest of organizations like the Geological Society of America, oceanographic institutions, and governments which are allocating considerable sums for such research.

Beginning three or four years ago, when announcement was made of findings in rather detailed hydrographic surveys on parts of the Atlantic coastal shelf, and also off the California coast and opposite the mouth of the Congo, efforts have been multiplied to secure closely spaced soundings in many marine areas. Steep-sided canyons with the dendritic distribution pattern that is characteristic of valleys on land are found in numerous places to descend below sea level as much as 9,000 feet, and submerged deltas representing at least two cycles of building are identified in connection with some of these thousands of feet below sea level. Some of the submerged valleys are rock-walled. Some are continuations of stream valleys on land, as for example, the Hudson, but others are not. The existence of some of the submerged valleys has long been known but little attention has been paid to them. Because evidences are increasingly numerous that these deeply submerged valleys were carved by running water above sea level, and because the valleys on the east and west coasts of North America and on the cast side of the Atlantic all appear to be contemporaneous, geologically rather recent results of erosion, it seems necessary to conclude that mean ocean level has fluctuated several thousands of feet with respect to the lands during or since the time of the last great Ice Age. Removal of water to make ice sheets and return of part of this water to ocean basins by melting of the ice however, are quantitatively very inadequate as explanation.

Methods of taking core samples, 12 feet or more in depth, of ocean bottom deposits have recently supplied materials from the Atlantic floor that by alternating succession of warm water and cold water foraminiferal assemblages in the sediments are thought to show the changing conditions due to glaciation and deglaciation in Pleistocene and Recent time. On the other hand, uniformity of sediments in cores from deep areas in the Pacific Ocean, some 300 miles from shore, indicate little fluctuation in conditions of sedimentation. These cores also give a measure of the rate of sedimentation in the places sampled.

A current subject of study and discussion among geomorphologists who are interested in oceanography is a reclassification of all known types of shorelines on a genetic basis.

Historical Geology.

An important set of volumes, now in process of writing and publication, representing the summary conclusions of one of the world's leading stratigraphers, A. W. Grabau, of Peking, China, is more than half completed. This work ('Palaeozoic Formations in the Light of the Pulsation Theory') reviews features of the geologic record in all parts of the world, and, with voluminous reference to paleontologic data, undertakes to trace the chief changes in relative emergence of the continents above periodically wide-spreading seas. Proposal is made to revise the names and limits of the major divisions of the geologic column and the corresponding divisions of geologic time to accord with evidences of great marine and continental 'pulsations.' Each pulsation period witnessed more or less widespread submergence of continents, followed by withdrawal of seas with accompanying erosion of lands. In spite of the soundness of most of Grabau's interpretation, the inertia of long usage of familiar old terms will not allow general adoption of this new classification and nomenclature, especially since some of the innovations among terms are not too well chosen.

The necessity for very careful segregation of paleontologic evidences according to the exact geologic horizons from which fossils may be collected, and the similar care that must be exercised in correlating rock strata on the basis of the order of succession, lithologic characters, and fossil contents, are well shown by a review of the famed and highly controversial 'Laramide problem,' reported in 1938 by E. Dorf. This problem, which is mentioned in almost all American textbooks of historical geology, has to do with the boundary between Mesozoic and Cenozoic deposits in the western interior of North America and bears on the geologic dating of the Laramide (or Rocky Mountain) mountain-building. Dinosaurs, the characteristic great reptiles of Mesozoic time, have been thought to be represented by skeletal remains in rocks that also yield typical floras of early Cenozoic time; this association could only mean contemporaneous existence of surviving dinosaurs and newly appeared, modern types of trees and shrubs. The 'transition beds' were considered by some geologists as Mesozoic and by others as Cenozoic. Sifting out by means of new field studies the actual occurrences of the fossil reptile bones and plant remains shows that the flora of the dinosaur-bearing deposits is actually quite distinct from that of other plant-bearing beds with which these had been confused, and it becomes possible to settle the position of the geologic boundary between these two eras, as well as to resolve certain questions about the age of crustal deformations in the Rocky Mountain region.

Studies of the age of rocks by means of measurement of accumulated lead produced by disintegration of radioactive minerals have been supplemented in late years by effort to define also the helium that is similarly produced by the disintegration. This latter method offers difficulties because of the mobility of the gas, but it is applicable to many rock samples where determination of radium-produced lead is not practicable. Recent independent remeasurements by a number of analysts have shown that previously published age determinations, secured from use of the helium method, are slightly too great. The revised helium scale shows ages of different rock samples ranging from 3 to 1,500 million years, and this accords well with results on other samples by the radium-lead method.

Paleontology.

Studies of fossils are of fundamental importance in the advance of geologic knowledge, and publications of 1938 giving results of paleontologic research are numerous. Mostly, however, these are highly technical reports that are without interest for the layman. This does not mean that the subject matter is devoid of interest, generally speaking, but rather that treatment is much too detailed, too greatly bound by requirements of scientific precision, and too universally given in specialized terminology to be understood or widely appreciated. It is unfortunate that this storehouse of information concerning the varied forms of past animal and plant life on the earth is so largely locked in the vault of forbidding, though often beautifully illustrated, publications.

Work on fossil plants that is specially worthy of notice includes a monograph on one of the oldest known types of conifers, named Lebachia (former Walchia), by the Swedish paleobotanist, R. Florin; these plants were widespread in cool, relatively dry lands of Upper Carboniferous and Permian time. One of the largest known floras of Jurassic age from eastern Greenland has recently been described by T.M. Harris. The most ancient of all known plants, possibly excepting bacteria, are varied sorts of algae from Proterozoic rocks. Those found in northwestern Montana, in the Glacier Park area, have recently been described by C. L. and M. L. Fenton.

Publications.

A few outstanding contributions to invertebrate paleontology recently published, are: monographs on certain families of fossil Foraminifera by J. A. Cushman; the remarkable succession of 'wheat grain' foraminifers called Fusulinids which are important zone fossils in Permian rocks of Texas, by C. O. Dunbar and J. W. Skinner; Cambrian and early Ordovician brachiopods, by E. O. Ulrich and G. A. Cooper; late Paleozoic Sectionid Clams (scallops), by N. D. Newell; and the complexly-organized chambered cephalopods called Ammonoids — Devonian by A. K. Miller, and Carboniferous-Permian by F. B. Plummer and G. Scott. Increasing attention to use of fragmentary fossil remains, representing various sorts of invertebrates, is proving valuable in many cases where other material is inadequate for correlation of beds. In 1938 there appeared the first two sections (Wenz: Gastropods; Bulman: Graptolites) of a projected 20-volume work on Invertebrate Paleontology, with authorship distributed among leading specialists of the world; published in Germany under guidance of O. H. Schindewolf, one of the issued sections is in English, another in German, and other divisions will use these languages or French, according to choice of authors. Of outstanding importance as a reference and advanced text, this series is intended to contain diagnoses and classification of practically all known genera of fossil invertebrates.

Among contributions to vertebrate paleontology, special notice is due several studies of fossil fishes, because greater relative advance in knowledge appears lately to have been made here than in other branches. One of these works (D. M. S. Watson: 'The Acanthodian Fishes') especially, fills a gap in furnishing information on the oldest and most primitive known gnathostomes, Silurian to Permian in age; these are treated as a group that is equal in rank to Pisces, the fishes proper. An enlarged and rewritten edition of W. B. Scott's 'History of Land Mammals in the Western Hemisphere,' which has been, since 1913, one of the most widely read books on Tertiary vertebrates, brings this important, well-illustrated treatise abreast of recent studies of fossil mammals.

Fossil Remains.

The Great Plains region of the central western United States, especially in parts of Nebraska and South Dakota, has furnished numerous skeletons and fragmentary remains of three-toed horses, camels, and other mammals of many kinds, now extinct, that roamed this country during the past 20 to 30 million years. Search for these fossils and studies of them by American paleontologists have made the evolutionary history of mammals much better known than would have been possible by work in other continents where remains of these animals are less abundant and not so well preserved. Recent researches in this field are resulting in a more exact determination of the geologic sequence of the mammalian faunas, which is important in studies of the evolution of individual stocks, in defining geographic or environmental modifications, and in reaching conclusions as to certain intercontinental migrations that are otherwise indicated by occurrence of closely-related animals in distant parts of the earth.

A discovery that is of intrinsic interest, and one that aids greatly in precise correlation of local deposits of fossil bones in the plains country, as well as in work to determine geologic structure that affects water-supply and that may indicate oil or gas occurrences in deeper rocks, is identification (M. K. Elias) of several zones of fossil grass seeds in the Tertiary deposits of Kansas and neighboring states. These are silicified minute fossils that are very widespread and in places very abundant. The fact that different kinds of these seeds mark various levels in the strata serves to define distinct ages of accumulation of the sediments and to furnish evidence of relative age of vertebrate bones found with them locally.

Further investigations of geologic evidence as to age of the 'Minnesota girl,' discovered by workmen on highway grading in 1931, indicates general agreement that this skeleton is about 20,000 years old and belongs near the beginning of man's advent in North America; this view is contested, however, by at least one glacialogist (Antevs). Human artifacts, estimated to be about 10,000 years old, were recently found in western Kansas, and evidences of early man as represented by the so-called Folsom culture, of about this age or older, are increasing by finds in several localities.

Petrology.

Very noteworthy in this branch of geologic science is the completion during 1938 of a monumental work by Albert Johannsen, emeritus professor of petrography at the University of Chicago, on the description and classification of the igneous rocks of the world. This is a treatise in four large volumes, 'A Descriptive Petrography of the Igneous Rocks,' that brings together practically all analyses in world literature giving chemical and mineralogical composition of igneous-rock specimens. Not least among difficulties confronting petrographers are the problems in making a truly satisfactory classification of the multitudinous types of rocks now known and the myriad names that are applied, mostly without system; these features are clearly shown in Johannsen's extremely valuable compilation and original contributions.

Another noteworthy publication in this field is a revision by A. K. Wells of the well-known textbook of the late F. H. Hatch, eminent British petrologist; this work, 'Petrology of the Igneous Rocks' presents a rational classification and simplified nomenclature that are to be commended, and in the latter part gives a clear account of igneous activity in the British Isles during geologic time. The first comprehensive treatment in English of the so-called petrofabric technique in study of rocks, developed in Germany by Sanger, has just been written by E. B. Knopf and E. Ingerson ('Structural Petrology'), and issued as a monograph of the Geological Society of America. Study of the orientation of mineral constituents in rocks permits determination of their structural history and has many important applications in theoretic and economic investigations.

For several years an increased attention has been given to the study of the sedimentary rocks, both from the standpoint of sedimentation and from petrographic research. This work is important in several lines of economic application and in stratigraphic interpretation, as well as in achieving better balance within the field of petrology as regards knowledge of the three main classes of rocks, igneous, sedimentary, and metamorphic. A well-written book by W. C. Krumbein and F. J. Pettijohn ('Sedimentary Petrology') that has just appeared fills a widely expressed want.

Geophysics.

A variety of instruments and methods are now successfully being applied to the determination of geologic features that are only partially known or undeterminable from surface work. Chief among geophysical tools are the seismograph, torsion balance, gravimeter (for direct measurement of gravity), magnetometer, and various sorts of electrical resistance or conductivity instruments. Subsurface delineation of such geologic features as contacts between rock formations, sheared zones, faults, occurrence and position of quartz veins, silicified zones, ore bodies, water-bearing sand, gravel or rock, and thickness of overburden above bed rock, is possible with varying degrees of precision. Work of this sort is being done mainly by private concerns but is also conducted on an increasing scale by Government agencies. Millions of dollars invested in geophysical work in American oil fields, combined with geologic surface and subsurface work, is responsible for continued discoveries of needed petroleum reserves. In South Africa, geophysical studies have revealed an 18-mile extension of the Rand gold-bearing 'reefs' and it is certain that if the gold content in this yet unworked area is approximately as rich as in the mined district the world's annual production of gold will be greatly increased. Magnetic methods, in exploration for iron ore have long been applied successfully in North America, South America, and Europe. Copper, nickel, lead, zinc, and gold are being found by use of electrical methods, and similar technique is showing the location of potable water supplies in arid regions of the United States, Spain, Africa, and Palestine.

A new and promising type of investigation in the search for old fields is soil analysis to determine presence of ethane or certain other hydrocarbons, with measurement of their very minute quantities. Locally concentrated occurrence of the hydrocarbons signifies slow seepage from underground reservoirs, which, in light of data as to distribution of hydrocarbon-rich samples and in some cases with consideration of electrical resistance anomalies, may be used to guide drilling.

Seismology.

The chief recent advances in seismology include: installation of several new stations and the modernization of old ones, development of earthquake information services, inauguration of a strong motion program providing records for studies of earth structure, work on ground vibration, study of the effect of regional anomalies in gravity on transmission time of earthquake waves, and special triangulation and leveling work in order to furnish observational-control data in regions subject to earthquakes (N. H. Heck). Several seismic stations equipped with identical instruments of high magnification that lately have been established in New England show a considerable number of local quakes for which the epicenters have been determined; the new data, supplemented by future records, are expected to throw light on problems of seismicity of the region. Data on some 300 reliably recorded 'deep focus' earthquakes, emanating from depths of 100 kilometers (about 61 miles) or more within the earth, indicate a certain periodicity of occurrence that is correlated with tidal stresses.

Structural Geology.

One of the most important recent contributions to knowledge of the nature of mountain building appears to be the discovery by V. Meinesz of extraordinary negative anomalies (gravity below normal, indicating less than average mass in the underlying earth crust) in the vicinity of island arcs, like that of the Lesser Antilles. A belt of strong negative anomalies is now known to extend eastward from Cuba, north of Haiti and Puerto Rico, around the outside of the Lesser Antillean arc, crossing Barbados and Trinidad, and continuing westward north of the South American coast to Colombia. This belt is interpreted as caused by the downbuckling of the upper crust, some 15 miles in thickness, which is reflected partly in a deep trough that is the topographic expression of the buckle and partly by a ridge (produced by upward pushing of weak materials) flanked by deep troughs. There is evidence of thrusting and overturning of rocks in an outward direction on both sides of the axis of the belt of negative anomalies, and volcanic activity, numerous earthquakes, and faulting especially characterize this belt. The observations indicate a mountain chain in process of development.