412 GEOLOGY OF DELAWARE COUNTY. 



There are several more exposures of serpentine in the County which require 

 no particular notice. Their size and location are exhibited on the Geological 

 map. 



There has now been brought in review before the reader all the underlying 

 rocks of the County, in fact every thing that has the name of rock in common 

 language. But in the language of the Geologist everj' deposit that constitutes 

 a part of the crust of our planet, however soft and earthy, is a rock, and such 

 deposits often form the most interesting subjects of his investigations. 



Besides the very recent alluvial deposits of mud and sand along the margin of 

 the river, which will require no particular notice, we have two distinct deposits 

 of clay, gravel, and sand, of diflerent ages, resting uncouformably on the rocks 

 that have been noticed, — the strata of the solid rocks having geuerally a dip ap- 

 proaching to vertical, while the distinctive lamina of these more recent beds do 

 not vary much from horizontal. On the Geological map of the State survey one 

 of these deposits is designated as " diluvium" the other has neither been laid 

 down on the map nor noticed in the text. It need hardly be mentioned that 

 both deposits were formed when the spots they occupy were submerged, consti- 

 tuting the bottom of a body of water of no great depth. 



The beds of one of these deposits occupy a portion of the eastern part of the 

 County above ■what has been called the " water shade," (and which might with 

 no impropriety be termed the " ancient river terrace,") while those of the other 

 are spread over nearly the whole space between the base of this terrace and the 

 river, the exception being on the margins of streams, where its loose materials 

 have been removed. 



It was not till about fifteen years ago that any such difiference was observed 

 by me between these upper and lower beds as would suggest a difference in 

 their age, and the conditions under which they were deposited. Such a differ- 

 ence does, however, exist, and it is of a character to elicit speculations in respect 

 to the remote geological condition of this section of country, and its passage 

 from that condition to its present, of the highest interest. 



In their general aspect the deposits have no striking difference, except the one 

 nearest the river presents a larger proportion of clay beds, and the quality of the 

 clay is much superior, being generally of the kind known as " brick clay." 



A close examination of the upper beds of gravel will show that the materials 

 composing them (mostly quartz) have been derived from the oldest rocks. No 

 such particular examination of these beds has been made as to justify a positive 

 denial of the existence in them of matter derived from rocks of a later formation, 

 but it may be asserted with confidence that if such matter does exist, it is in 

 excessively minute proportions. Of the upper beds, only the very remnants 

 remain. To such a great extent have they been washed away, that they are only 

 found of any great thickness, at a few high points in the district of country over 

 which they are spread, such as Sandy Bank near Media, and the Presbyterian 

 meeting-house in Marple. For a considerable space along the margins of the 

 streams throughout the whole district, all, or nearly all of this deposit has been 

 washed away ; the soil there being formed from the decomposition of the under- 

 lying rocks in place. 



Now, the lower beds, instead of being free from pebbles of the later rocks, in 

 some places abound in them. These are mostly from the disintegration of sand- 

 stone rocks; and among them, water- worn blocks of sandstone of a considerable 

 size are frequently found. It is not difficult to point to the parent rock, that by 

 its disintegration, furnished the sandstone pebbles and water-worn blocks to 

 these lower beds, but it is not so easy to explain why a supply of the same 

 materials was withheld from the gravel beds of the upper deposit. 



But one explanation has occurred to the author, and that rests upon the suppo- 

 sition that this section of our continent was gradually upheaved from its former 

 submerged condition. "When the upper beds were formed, no disintegration of 

 the sandstone rocks had occurred, or if it had, there was no current to transport 

 the resulting materials in the direction of the upper beds. They therefore 

 necessarily consist of the disintegrated materials of the older rocks upon which 

 they rest, or of the same class of rocks fouud iu the immediate vicinity. 



