1375.] 101 [Rogers. 



place and scattered, consist of a gray silici-ous rock or quartzite, 

 seemingly referable to some member of the primordial group, of 

 which a remnant is exposed in southeastern Massachusetts, and per- 

 haps a larger extent is concealed by drift, and which probably at 

 one period spread northeastward over extensive areas now covered 

 by the sea. 



Art. II. On the Gravel and Cobble-stone Dejyosits of Virginia and the 

 Middle States. 



The surface deposits here referred to are extensively exposed in 

 many parts of the belt which marks the junction of the older rocks 

 with the tertiary and upper secondary formations in the Middle 

 States. These deposits, especially in the great river valleys and 

 adjoining slopes, as at Richmond and Washington, consist chiefly cf 

 layers of quartz gravel, like the surface gravel of the adjoining pri- 

 mary region, and of larger smoothly rolled masses derived from the 

 silicious slates, quartzites and sandstones of remoter tracts lying to- 

 the west and northwest, mingled and interstratified with ferruginous 

 sands and clays, which impart to the mass a more or less reddish. 

 color. 



In most localities, the larger pebbles are found in the upper part of 

 the deposit, often strewing the surface thickly where the finer matter 

 has been removed either by natural erosion or in the progress of 

 improvement, as may be seen at numerous exposures in and around 

 Washington. In other cases, as at Alexandria and at Richmond, the 

 cobble stone deposit is usually overlaid by stratified sand and gravel 

 of considerable thickness. It is from these- sources that the cities of 

 Richmond, Washington, Baltimore and Philadelphia, have been sup- 

 plied with the paying materials at one time so generally in use. 



In a pile of such paving stones in Richmond, Virginia, many 

 years ago, I found a large pebble of compact vitreous sandstone, 

 containing distinct impressions of Scolithas linearis, the well-known 

 characteristic fossil of the Primal or Potsdam formation, having its. 

 nearest outcrop on the western side of the Blue Ridge. In subse- 

 quent observations, especially those recently made in and around 

 Richmond, Washington and Georgetown, I have found that a con- 

 siderable proportion of this pebbly or cobblestone deposit consists of 

 fragments of the harder silicious Paleozoic rocks, and has therefore 

 been derived from the Appalachian belt. Indeed, so common are 

 the fossiliferous fragments, that an observer can hardly fail to clis- 



