192 National GeograpJiia Magazine. 



the whole series of bedded formations was deposited, and the 

 basal sandstone that is generally associated with it. Wherever 

 we now see these harder rocks, they rise above the surrounding 

 lowland surface. On the other hand, the weaker beds are the 

 Cambrian limestones (Trenton) and slates (Hudson River), all 

 the Silurian except the Medina above named, the whole of the 

 Devonian — in which however there are two hard beds of subordi- 

 nate value, the Oriskany sandstone and a Chemung sandstone and 

 conglomerate, that form low and broken ridges over the softer 

 ground on either side of them — and the Carboniferous (Mauch 

 Chunk) red shales and some of the weaker sandstones (Coal 

 measures). 



6. Former extension of strata to the southeast. — We are not 

 much concerned with the conditions under which this great series 

 of beds was formed ; but, as will appear later, it is important for 

 us to recognize that the present southeastern margin of the beds 

 is not by any means their original margin in that direction. It 

 is probable that the whole mass of deposits, with greater or less 

 variations of thickness, extended at least twenty miles southeast 

 of Blue Mountain, and that many of the beds extended much 

 farther. The reason for this conclusion is a simple one. The 

 several resistant beds above-mentioned consist of quartz sand 

 and pebbles that cannot be derived from the underlying beds of 

 limestones and shales ; their only known source lay in the crys- 

 talline rocks of the paleozoic land to the southeast. South Moun- 

 tain may possibly have made part of this paleozoic land ; but it 

 seems more probable that it was land only during the earlier 

 Archean age, and that it was submerged and buried in Cambrian 

 time and not again brought to the light of day until it had been 

 crushed into many local anticlines* whose crests were uncovered 

 by Permian and later erosion. The occurrence of Cambrian 

 limestone on either side of South Mountain, taken with its com- 

 pound anticlinal structure, makes it likely that Medina time found 

 this crystalline area entirely covered by the Cambrian beds ; 

 Medina sands must therefore have come from farther still to the 

 southeast. A similar argument applies to the source of the 

 Pocono and Pottsville beds. The measure of twenty miles as the 

 former southeastern extension of the paleozoic formations there- 

 fore seems to be a moderate one for the average of the whole 

 series ; perhaps forty would be nearer the truth. 



* Lesley, as below. 



