120 PROCEEDINGS OF THE ACADEMY OF [March, 



Lithologically, this quartzite is in every way identical with 

 the Shawangunk, which forms Blue Mountain twenty miles away 

 to the north, and with the Green Pond conglomerate of the same age, 

 occurring in the highlands of northern New Jersey. Not only 

 does this similarity cover the general features of the rock, but it 

 extends down to such small details as the extent of the silicification 

 visible under the microscope, the inclusion of the gray slate flakes, 

 and the occasional presence of copper sulfides (chalcocite?) in 

 minute disseminated grains, just as occurs at the Pahaquarry copper 

 mine on Blue Mountain, eight miles northeast of the Delaware 

 Water Gap. And since there is no other formation anywhere 

 in the region of at all similar lithologic character, there w^ould seem 

 to be no reasonable doubt as to the correctness of this interpretation. 



In addition to these quartzite pebbles, limestone fragments 

 are often present in the conglomerates, locally forming almost the 

 only constituent of the rock. These are usually less well rounded 

 than the quartzite, and, in fact, are often so angular that the rock 

 should be termed a breccia. When exposed to atmospheric agencies 

 they have usually weathered out, leaving a peculiar-looking cellular 

 rock. Nothing has been observed to indicate that these limestone 

 pebbles are of different type from the Paleozoic (Cambrian and Lower 

 Ordovician) strata exposed in the valleys to the north; in fact, 

 streaks of the black chert so frequently present in these beds have 

 been noted in some of the pebbles. Along with the limestone peb- 

 bles are also abundant flakes of a greenish schistose material, which 

 resembles the sericite partings developed in the limestones in many 

 places, occasional beds of the conglomerate being made up of nothing 

 but overlapping chips of this schist. 



Again in some places, gneiss pebbles are present in considerable 

 numbers, several of the types now exposed in the hills to the north 

 being represented. These, like the limestone fragments, are only 

 imperfectly rounded, and they have also weathered on the surface to 

 some extent, although perfectly fresh when seen in recent artificial 

 exposures, as along the trolley line south of Monroe, on the Dela- 

 ware River. 



On descending the hill slopes it is found that the pebbles in the 

 soil become gradually fewer in number, and finally give way to 

 frost-shattered shale fragments, as roughly indicated on the 

 map by the small circles. There is practically no decrease in 

 the sizes of the pebbles going outward from the centers of the hills, 

 and absolutely nothing like a gradation from the conglomerates 



