742 HISTORICAL GEOLOGY. 



have gone down 3000 feet through sandstone alone ; and seldom are the inter- 

 calated beds of limestone and shale of sufficient extent to mark a horizon 

 and serve as the means of measuring the thickness. At 'New Haven, Conn., 

 an artesian boring was carried down 4000 feet through porous sandstone 

 without finding variation enough in texture to get a supply of water. 



The layers often have a cross-bedded structure and other evidences of 

 strong currents. In many regions they are here and there covered with 

 ripple-marks, mud-cracks, raindrop impressions, footprints of Rej)tiles and 

 Amphibians ; the fine shales with tracks of Insects and Crustaceans — facts 

 which indicate temporary exposures above the water level of great sand-fiats 

 and mud-flats. A slab from Greenfield, Mass., a dozen feet long, now in the 

 Yale Museum, is covered throughout with deep impressions of raindrops — 

 the work of a short large-drop shower. The impressions are a little elliptical 

 so as to register the direction of the accompanying wind. Besides this, two 

 lines of large three-toed tracks cross the slab, and those of the longer line 

 are dotted by the raindrops, showing that a biped reptile had passed that 

 way before the shower began. 



The material of the sandstones and conglomerates, exclusive of the 

 calcareous, is almost solely such as would be afforded by the wear of granite, 

 gneiss, mica schist, syenyte, and other crystalline rocks of the neighboring 

 hills or mountains ; and the amount of mica and other ingredients and kinds 

 of rock material vary with the kind of rock in the adjacent hills. Several 

 examples of this are mentioned by Emerson, Pontaine, and others. The 

 feldspar is usually fresh and undecomposed, and well mixed with the quartz, 

 showing no evidence of any assortment of the ingredients by beach action. 

 The ingredients are often in proportions fitted to make granite again by 

 subjection to metamorphic action. Mica is sparingly present except where 

 mica schists exist on the border of the areas. There are also limestone con- 

 glomerates in regions where Cambrian or Lower Silurian rocks exist along 

 the border ; and occasionally stones of a quartzose conglomerate derived from 

 a Cambrian sandstone or quartzyte. 



The coarsest conglomerates consist of stones of all sizes up to five feet 

 across, and usually occur along the eastern or western border of an area. 

 In Montague, Mass., east of the Connecticut, on the eastern border of the 

 area, and in Branford, Conn., some of the bowlders are three feet across. 

 Similar cases exist on the west border of the western area in New Jersey, 

 Virginia, and ISTorth Carolina. In the Pittsylvania belt, the larger stones are 

 four to five feet in diameter. Near Point of Pocks, Md., the stones are of 

 Paleozoic limestone, and some are two feet through ; the finer variety of this 

 limestone conglomerate is the " Potomac pudding-stone marble." 



The Coal-measures in the Pichmond basin and Virginia, and in North 

 Carolina, consist of beds of shale and sandstone with thick beds of good coal. 

 In the Pichmond area there are two to eight coal-beds, and the main bed is 10 

 to 40 feet thick ; but they include some thin dividing layers of sandstone and 

 shale. The Coal-measures are situated within 250 to 500 feet of the bottom 



