Following the founding of the Journal in 1818 there is observable a growth in the quality and detail of geological mapping. Dr. Aiken, professor of natural philosophy and chemistry in Mt. St. Mary’s College, published in the Journal in 1834 (26, 219) a vertical section extending between Baltimore and Wheeling, a distance of nearly 250 miles, on a scale of about 7 miles per inch. The succession of rocks is carefully shown and the direction of dip, but no attempt is made to show the underground relations, the stratigraphic sequence, and the folded structures which are so clear in that Appalachian section. The text also shows that the author had not recognized the folded structure. Furthermore, where the folds cease at the Alleghany mountain front, the flat strata are shown as resting unconformably on the folded rocks to the east.
R. C. Taylor, geologist, civil and mining engineer, was from 1830 to 1835 the leading student of Pennsylvanian geology as shown by the publication in 1835 of four papers aggregating over 80 pages in the Transactions of the Geological Society of Pennsylvania. His work is noticeable for accuracy in detail and no doubt was influential in setting a high standard for the state geological survey which immediately followed.
H. D. and W. B. Rogers have been given credit in this country, and in Europe also, as being the leading expounders of Appalachian structure. Merrill speaks of H. D. Rogers as unquestionably the leading structural geologist of his time.[[85]] To the writer, this attributed position appears to be due to his opportunities rather than to scientific acumen. The magnificent but readily decipherable folded structure of Pennsylvania, the relationships of coal and iron to this structure, the considerable sums of money appropriated, and the work of a corps of able assistants were factors which made it comparatively easy to reach important results. In ability to weigh facts and interpret them Edward Hitchcock showed much more insight than H. D. Rogers, while in the philosophic and comprehensive aspects of the subject J. D. Dana far outranks him.
H. D. Rogers in his first report on the geological survey of New Jersey, 1836, recognizes that the Cambro-Silurian limestones (lower Secondary limestones) were deposited as nearly horizontal beds and the ridges of pre-Cambrian gneiss (Primary) had been pushed up as anticlinal axes (p. 128). He also clearly recognized the distinction between slaty cleavage and true dip as shown in the Ordovician slates (p. 97). Between 1836 and 1840 he had learned a great deal on the nature of folds as is shown in his Pennsylvania report for 1839 and the structure sections in his New Jersey report for 1840.
R. C. Taylor, who had now become president of the board of directors of the Dauphin and Susquehanna Coal Company, published in the Journal in 1841 (41, 80) an important paper entitled “Notice of a Model of the Western portion of the Schuylkill or Southern Coal Field of Pennsylvania, in illustration of an Address to the Association of American Geologists, on the most appropriate modes for representing Geological Phenomena.” In this paper he calls attention to the value of modeling as a means of showing true relations in three dimensions. He condemns the custom prevalent among geologists of showing structure sections with an exaggerated vertical scale with its resultant topographic and structural distortions. Taylor was widely acquainted with the structure of Pennsylvania, Maryland, and Virginia.
Nature of Forces Producing Folding.
In 1825 Dr. J. H. Steele sent to Professor Silliman two detailed drawings and description of an overturned fold at Saratoga Lake, New York. As to the significance of this feature Steele makes the following statement (9, 3, 1825):
“It is impossible to examine this locality without being strongly impressed with the belief that the position which the strata here assume could not have been effected in any other way than by a power operating from beneath upwards and at the same time possessing a progressive force; something analogous to what takes place in the breaking up of the ice of large rivers. The continued swelling of the stream first overcomes the resistance of its frozen surface and having elevated it to a certain extent, it is forced into a vertical position, or thrown over upon the unbroken stratum behind, by the progressive power of the current.”
So far as the present writer is aware this is the first recognition in geological literature of the evidence of a horizontally compressive and overturning force as a cause of folding.
To E. Hitchcock belongs the credit of being the first to describe overturning and inversion of strata on a large scale, but without clearly recognizing it as such. In western Massachusetts metamorphism is extreme in the lower Paleozoic rocks in the vicinity of the overthrust mass of Archean granite-gneiss which constitutes the Hoosic range. The Paleozoic rocks of the valley to the west are overturned and appear to dip beneath the older rocks. Farther west the metamorphism fades out and the series assumes a normal position. Such an inverted relation, up to that time unknown, is described in 1833 as follows by Hitchcock in his Geology of Massachusetts (pp. 297, 298):