PENNSYLVANIA. 473 



of Hudson River age. (See also Am. Jour. Sci., 1880, (3) XIX., pp. 

 413, 414.) 



From the above it will be seen that Mr. Hall makes his first and 

 second grotips — which, however, he seems to think may perhaps 

 belon<^ to<'ethcr — to be older than the Potsdam Sandstone, which over- 

 lies them unconformably. None of the rest of his groups can, there- 

 fore, be of Azoic age. This is a result very different from that attained 

 by Dr. Hunt and others, who have made most of this region pre-Silu- 

 rian. But Mr. Hall's views seem to have a substantial basis of obser- 

 vation, while Dr. Hunt's cannot be regarded as anything more than 

 theories based on lithological resemblances. How fanciful these are 

 may be inferred from the contradictory statements of that author him- 

 self in regard to the rocks in question.* 



By way of illustrating the difficulty of separating the lower formations 

 from each other in the crystalline belt of Pennsylvania, it may be well 

 to add one or two extracts from the latest publication of the present 

 Survey of that State ; — " The Geology of Chester County, after the Sur- 

 veys of Henry D. Rogers, Persifor Frazer, and Charles E. Hall. Edited 

 by J. P. Lesley [State Geologist], C^ 1883." In this publication two 

 maps are given, one of which embodies the conclusions reached by Pro- 

 fessor Rogers, the other those of Mr. C. E. Hall, the region embraced in 

 these maps being essentially the same, and forming part of Delaware 

 and Chester Counties. A comparison of these maps will show how 

 utterly unlike they are ; but perhaps it will be better to quote Profes- 

 sor Lesley's own language on this point. He says (l. c, p. viii.) : — 



" It is impossible to imagine a greater contrast than between these two illus- 

 trations of opposite views. The great regularity of Mr. Rogers' belts, the utter 

 irregularity of Mr. Hall's areas, strikingly exemplify the difference between the 

 conclusion arrived at, in a difficult region hke this, by the earlier geologist 

 who made everything bend to his theory of parallel overturned anticlinals and 

 synclinals, and the observations of the later geologist who is fettered by no 

 such theory, but is perhaps quite as strongly influenced by a different senti- 

 ment, viz. : that the Azoic formations spread out over one another with mod- 

 erate inclinations unconformably." 



Furthermore, the following may be quoted from the same source, as 

 illustrating the facts that even the later geologists — those, namely, 

 who have worked longest and most perseveringly in this difficult field — 

 have not been able to arrive at concordant results. Professor Lesley 

 remarks (/. c, p. 34) : — 



• See ante, pp. 469, 470. 



