1879.] Ol'O [Stevenson. 



November of 1877, when a Mytiloid shell, which is associated with Ortho- 

 ceras and other forms of marine life elsewhere in the district, was found 

 inside the cast of a Lepklodendron, where it had been preserved by the 

 superior hardness of the material in which it was imbedded. 



If fossils were ever present in the detritus covering these benches, they 

 have been dissolved out by ordinary water or by water carrying carbonic 

 acid in solution. As the coating is merely incoherent sand, all animal 

 remains would be leached out of it in a comparatively short time, while 

 fragments of wood, which occasionally occur, would be unaffected. In- 

 stances of this kind are common enough, so that they need not be referred 

 to in detail. 



An objection to the marine origin of these benches may be found in the 

 existence of fresh water forms on the summit of Chestnut Ridge. That 

 such forms do occur there seems to be altogether probable from the testi- 

 mony of several persons, but I have been wholly unable to discover the 

 localities, although I have made diligent search. The shells obtained at 

 the summit of that ridge near the National road could not have been 

 numerous, for close examination of the sand on several occasions yielded 

 not even a fragment. 



If remains of fresh-water mollusks do occur on the crest, they mark 

 places once occupied by ponds such as those now to be seen at a little way 

 north from the National road and very near the summit. The presence of 

 the great Lower Carboniferous limestone is exceedingly favorable to the 

 growth of mollusks. This explanation is the more acceptable because 

 according to the statements made, the specimens are too well preserved to 

 admit of the supposition that they had been entombed for any considerable 

 length of time. They still retain the epidermis. Such being the case their 

 presence has no bearing whatever on the age or origin of these benches. 



If these benches are old beach lines, as seems by no means improbable, 

 they show that, at some period since the glacial time, the sea has covered 

 the greater part of the continent, certainly submerging the present summit 

 line of the Alleglnuiies in Pennsylvania. This would confirm the surmise 

 offered by Dr. Hitchcock in his " Illustrations of Surface Geology," when 

 discussing the sea-beaches found along the streams of New England. 



The lowest of these benches is at 1100 feet above tide. This statement 

 ho'ds good only for the extreme north-western part of the area examined 

 In the Ligonier Valley the lowest horizontal bench is at a greater altitude, 

 so, also, in southern Somerset county. Of necessity the horizontal benches 

 ceased when Uie drainage was re established, that is when the line of sub- 

 mergence sank below the stream beds previously existing. When the 

 water sank below- 1100 feet, it fell below that level in this region and the 

 lower series of terraces began to form ; but further west and south-west 

 where the elevation of the country is less than within the area described, 

 horizontal benches should be found at altitudes less than 1100 feet above 

 tide, possibly down to within a very little way above the tide-level. 



