1880.] -^^1 [White. 



finally the period culminated and ended by the spread of tlie Homewood 

 Limestone as an almost universal covering over Western Pennsylvania. 



After this final castastrophe scarcely any of the life forms that had exist- 

 ed before the accumulation of the Sharon Conglomerate remained, though 

 the process of extinction and change had been progressive toward the south 

 from the beginning. 



On this theory only, so far as I can see, can we explaiu the results at 

 which Mr. Piatt of the Survey Corps seems to have arrived from the study 

 of the rocks to the south-east in Jefferson, Indiana and Ai'mstrong coun- 

 ties, viz : that the rocks of No. XI (Subcarboniferous) extend up to the 

 base of the Hometoood Sandstone, and that it is the only member that can 

 be properly placed in No. XII. 



In such localities where Subcarboniferous conditions seem to have ex- 

 isted until the epoch of the Homewood Sandstone, or top member of the 

 Conglomerate series, it w^ould certainly be unwise to include any lower 

 beds in this series, and yet it seems to me equallj^ wrong to draw the line 

 along the base of the Homeicood Sandstone in such a case and extending it 

 over wide areas, call everything below it Subcarboniferous. 



It is a question of time against conditions. If it be right to call all rocks 

 Siibcarbomferous that were formed everywhere and under anj"^ surroundings 

 until Subcarboniferous conditions had closed everywhere then it would 

 undoubtedly be correct to draw the line squarely at the base of the Home- 

 wood Sandstone ; but under the broader view that discards cataclysmal 

 changes in the Earth's history, and recognizes the now well proven fact 

 that almost all great changes have been gradual and progressive, and that to 

 be so they must have had a beginning somewhere in the midst of widely 

 differing conditions, recognizing this principle it seems to me there should 

 be no hesitation about extending the line downward from the base of the 

 Homewood Sandstone, as we proceed north, under the feather edges of 

 these northern Conglomerates, until we reach the base of the Sharon Con- 

 glomerate, even if this latter stratum were coetaneous with the deep sea 

 that prevailed along the south line of Western Pennsylvania during the 

 Chester limestone epoch, for all these conglomeratic sandstones, from the 

 base of the Sharon to the top of the Homewood, were formed under similar 

 conditions, and the Subcarboniferous sea at no time retui'ned northward to 

 restore the life forms which the appearance of tlie Sharon Conglomerate 

 had driven away forever. To do otherwise than this would be equivalent 

 to classifying the marine deposits which aro now taking place along the 

 coasts of this country and Europe with the Cretaceous, because perchance 

 Cretaceous life and conditions may now exist in the deep bed of the At- 

 lantic. 



I have discussed this matter somewhat at length, hoping to throw some 

 light upon and help to harmonize the hitherto conflicting results of all who 

 have undertaken the study of this question. It is certainly a difficult sub- 

 ject to deal with, but it seems to me that we have now got hold of some of 

 the threads at least, out of whicli the true history of the fabric can ])e 

 woven. 



PROC. AMER. PHILOS. SOC XTX. 107. /.. PRTNTEr JANUARY 18, 1881. 



