Mathews — Structure of the Piedmont Plateau. 157 



The generalization reached by Williams in this paper is one 

 of far-reaching importance and marked his ability to gain 

 clean-cut conceptions of intricate problems from the most 

 meager investigations. The writer has had occasion to re-ex- 

 amine most of the areas studied by Williams and has been 

 impressed with the grasp of the subject and accuracy of the 

 conclusions drawn when judged by the amount of information 

 at his disposal. It should be borne in mind, however, that at 

 the time this paper was written and even up to the date of 

 Dr. Williams' death, only a small portion of the Piedmont 

 in Maryland had been mapped by the topographers of the U. S. 

 Geological Survey and not all of this had been studied in 

 detail geologically. The generalizations drawn were based 

 upon rapid reconnaissance driving-trips across the Plateau 

 rather than upon detailed examinations carried on throughout 

 the region. 



Bearing these facts in mind, it is no serious criticism to 

 Williams when the writer asserts that the detailed work of the 

 last ten years, conducted with greater opportunities and with 

 the aid of topographic maps, shows that the facts on which 

 Williams based his conclusions are not proven. Taking up 

 the five points regarded by him as conclusively proving that 

 a time break exists between the eastern and western rocks, 

 the writer would call attention to the fact that the "fan-like 

 divergence of dip from a central vertical axis" does not seem 

 to be a correct interpretation of the structural lines found 

 within the region, which, on the contrary, indicate that there has 

 been a series of folds rather than a single syncline or fan-like 

 structure. The apparent occurrence of such a structure is due 

 partly, no doubt, to the confusion of true bedding planes with 

 those due to schistosity. The abrupt contact, such as Williams 

 seemed to find between the semi-crystalline and highly crys- 

 talline rocks of the Piedmont, apparently does not exist with 

 the sharpness formerly supposed, since Keith's work in Carroll 

 and How r ard counties has broken the sharp line drawn by 

 Williams into an intricate series of intertonguing areas. The 

 contact between the so-called phyllites and the gneiss as drawn 

 by Williams is, in most cases, a contact between the phyllites 

 and the mica-schists and gneisses of Hudson age rather than 

 with the banded gneisses believed to represent pre-Cambrian. 

 The contact here, judging from the experience of more recent 

 workers in the region, is not abrupt but is usually obscured by 

 the similarity of material or the likeness of soil resulting from 

 the mica-schists and banded gneisses. 



The fact that the igneous rocks of the eastern area when in 

 close proximity to the slates or schists show little metamor- 

 phism, may be explained by the fact that within the Maryland 



