22 THE GEOLOGIST'S TKAVELIKG HA:NT>-BOOK. 



5-8. SILURIAN (OR UPPER SILURIAN) AGE. 



5 a. Medina. The lower member of this formation is a pebbly sandstone 

 or grit called the Oneida conglomerate, being the same as the Shawangunk 

 conglomerate. The upper member is called distinctively the Medina sandstone, 

 and is usually a red or mottled argillaceous sandstone. 



1. The Oneida conglomerate in NewYorkis composed of quartz pebbles rarely 

 exceeding three-fourths of an inch in diameter, and of white or yellowish quartz- 

 sand. In some localities there is some interposed greenish shale. The source of its 

 materials was to the south, the rock being 500 feet thick in the Shawangunk 

 Mountain at Wurtsburg, on the N.Y. & Os. Mid. R. R, and 1000 feet thick in some 

 parts of Pennsylvania and Tennessee. The greatest thickness of the Oneida in 

 the eastern part of New York is 30 to 40 feet, but in the western part the 

 same place is occupied by a gray quartzose sandstone, fine grained and compact. 

 Passing upwards, the gray sandstone intermingles with the Medina sandstone, 

 which, in its lower parts, differs chiefly in color. The red color of the Medina 

 sandstone seems to be partially communicated to the gray below, which is often 

 striped and spotted with red. There is, lithologically, no very strong line of 

 demarcation between the two rocks. The oxide of iron, the red coloring matter 

 of the upper member, has been transfused through the material of the lower as 

 far as its particles could find admittance. The flagstones in the side- walks of 

 Buffalo and Rochester, of a white color clouded with red, are of this formation . 



In New Jersey the gray sandstone formation consists of a thick series of hard, 

 white and whitish gray siliceous rocks, of various degrees of coarseness, from that 

 of a fine grained, pure sandstone to that of a quartzose conglomerate with 

 thickly-set pebbles averaging half an inch in diameter. This is the summit of the 

 long, straight mountain ridge called the Kittatinny or North Mountain, extending 

 from near the Hudson River into Virginia. 



In Pennsylvania the Oneida conglomerate is a compact, greenish-gray, massive 

 sandstone, containing in many places thick beds of siliceous conglomerate, and 

 the Medina sandstone proper is a thick mass of alternating red shales and red and 

 gray earthy sandstones . It is the North Mountain of the great Cumberland valley. 



At the Delaware Water-Gap the whole mass of Oneida and Medina consists 

 of seven massive plates of coarse sand and conglomerate, separated by more 

 argillaceous layers from each other. Going west, the number, according to Prof. 

 Lesley, is reduced to five, and finally in Middle Pennsylvania to two, each of them 

 very thick, and making its own mountain-crest when the dip is vertical, while the 

 intermediate softer red mass forms a little valley between the crests. The whole 

 formation is about 1,900 feet thick. When the dip is gentle, the Oneida 

 makes a beautiful lofty terrace upon the flank of the mountain, the crest of which 

 is always made by the Upper Medina. Traced southward through Virginia into 

 Tennessee , this formation gradually thins away to 50 feet, as seen west of Knoxville. 



2 . The Medina sandstone proper succeeds the gray sandstone, there being no 

 definite line of division between them. In this rock is found the Fucoides Harlani 

 affording a positive character whereby to recognize it in the series . This sandstone 

 is almost invariably of a red color, generally a brown-red, more rarely variegated 

 light red and yellowish, and in a few rare instances of a light or whitish color, 



