818 The American Naturalist. [October, 
Mr. Walcott began the investigation in York County, where he de- 
termined the stratigraphic position of the Chickies quartzites and the 
York shales which are subjacent to the Lancaster (York Frazer) lime- 
stone. Paleontologic evidence shows them to be of Lower Cambrian 
age. The fauna of the main body of the limestone of York County, as 
shown by collections from three separate localities, is Cambrian. These 
localities indicate respectively, an Upper Olenellus zone, a horizon 
between the Lower and Middle Cambrian, and a lower horizon of the 
Middle Cambrian. 
The discovery of Lower Cambrian fossils in the compressed syncline 
of limestone in Lancaster County, south of Columbia, indicates that the 
limestone on the west side of the river is of the same geologic age, and 
that the shales and schists beneath it are of Lower Cambrian age. 
Mr. Walcott states that in York County there is no sedimentary 
rock other than the mesozvie new red sandstone—of later age than the 
Cambrian, unless it be the Peach Bottom slates and chlorite-schists of 
the southeastern corner of the country. He also thinks it probable from 
the closely related structure of Lancaster County that all the Lancas- 
ter limestones will fall within the Cambrian, unless it be some portions 
of the upper series, which may pass into the Ordovician. He applies 
this generalization to the entire extension of this series of limestone 
northeastward to the Delaware. 
All of the quartzites that have been referred to the Potsdam neces- 
sarily fall into the Lower Cambrian, as they are beneath the limestones. 
The South Mountain chain, as stated by Professor Lesley and Dr. 
Frazer, consists of two groups of rocks, a quartzite and an orthofelsite 
series, the latter being considered the superior series. Mr. Walcott’s 
investigations lead him to a different interpretation of the geologic 
structure of the mountain and the relations of the rocks composing it. 
He finds that the “ orthofelsite ” is in reality the lower series, and that 
the complicated structure of the mountain arises “ partly from folding, 
but more largely from the westward thrusts of masses of strata along 
the lines of fault of a low hade. This westward thrusting on the fault 
plane, complicated by previous folding of the strata, leaves masses of 
the subjacent, pre-Paleozoic rocks resting in various places on differ- 
ent members of the Lower Cambrian series, and also appears to inter- 
bed the quartzites and schists of the Cambrian in the schists, eruptives, 
etc., of the Algonkian.” 
The following are the concluding remarks in the section on Corre- 
lation : 
