272 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



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The adjoining 



of this great Piedmont belt, and their equivalents 



farther north in New England, have, until recently, 

 J been usually classified as Arehean. Veryeareful and 

 detailed mapping is now, however, resulting in their 

 subdivision, and in the identification of some as met 

 amorphic Paleozoic strata, others as foliated erupt- 

 l ives, while still others belong really to the pre Cam- 

 brian ages (Algonkian or Arehean). It is to the 

 detailed study of this Held in Marylandand northern 

 Virginia that the attention of the writer has of late 

 j years been directed. Some of his more general eon- 

 elusions in regard to the structure of the Piedmont 

 plateau have been communicated in a paper to the 

 Geological Society of America^ The more im- 

 portant of these, as far as they relate to the region 

 to be passed over, may be summarized as follows: 



The Piedmont plateau is divisible into an eastern 

 highly crystalline and a western semicrystalline 

 portion. The former consists of gneisses and holo- 

 crystalline mica-schists, quartzites and marbles, con- 

 taining an abundance of more or less dynamically 

 metamorphosed eruptive masses. All of these rocks 

 have a prevailing NHB. strike and a westerly dip. 

 The western portion, on the other hand, is composed 

 of partially metamorphosed sedimentary strata (seri- 

 cite and chlorite schists, ottrelite schist, phyllite and 

 limestone) and is nearly free from ancient eruptives. 

 The strike of these rocks conforms to that of the 

 eastern portion, but their dips are prevailingly to 

 the east. In spite of apparent conformity, and even 

 indications of transitions between these two portions 

 of the Piedmont region, they are separated by a 

 great time-break and unconformity. 



The easterly dips on the west, and the westerly 

 dips on the east, together with the nearly vertical 

 strata between, produce a radiating or fan structure, 

 and the axis of this fan is not coincident with the 

 contact between the crystalline and semicrystalline 

 portions. The thickness of either series of rocks, as 

 indicated by their present dips, would be so vast 

 that one must assume that the same beds are 

 repeated over and over again by tightly compressed 

 folds or thrusts, 

 section (Fig. I) made along the line of railroad between 



