Frazer.l 4U.Z [Dec. 4, 



and the schists below It to the quartzite 4400 feet thick. The same heds 

 measured by me in Lancaster county only amounted to 3400 feet. These 

 beds, therefore, thicken 1000 feet in the twelve miles which intervene 

 between this section and the city of Lancaster, and ot this thickening 400 

 feet belong to the schists below the limestone and 600 leet 1o the limestone 

 itself and its included schists.* The limestone, of which numerous 

 analyses will be found in Reports C, CC, CCC, M and MM, is dolomitic, 

 that is to say, it is a carbonate of lime, containing varjing amounts of 

 carbonate of magnesia. There is also some ground for believing that two 

 kinds of limestone are represented, each having its own peculiarities of 

 physical structure. It was noticed in many cases that two kinds of lime- 

 stone were often exposed in the same quarry, and that they usually 

 showed slight variations ot dip. One, which was apparently the elder, 

 was of a buff or grayish color, and less marked stratification ; the other 

 blue, with white streaks and spots of lighter colored limestone (often 

 calcite). One case was recorded where, in a contact between the two, 

 pebbles of the buff were found in the blue. There seems no doubt that 

 the great mass of limestone now under consideration w T as formed subse- 

 quently to the quartzite, arjd at about the epoch oftheCalcilerous Sand-rock 

 of New York and before the Trenton, or in other words in the Canadian 

 epoch of Dana. But no fossils were found in the county to settle the 

 question. The portions of the beds connecting the limestone near New 

 Market with that of York (a connection which doubtless exists), is 

 covered up by the beds of the Mesozoic. Those which once connected 

 that of Wrightsville with that near Prospect has been washed away in the 

 general planing down of Ihe surface by erosion. The limestone is Indi- 

 cated in the map by white line blocks through the dark green. 



The Mesozoic Rocks in York County. 



None of the numerous members of Mesozoic rocks is known to be rep- 

 resented but the groups of sandstones and shales known as the "New 

 Red Sandstone," and sometimes the "Triassic Sandstone." 



There are many puzzling questions which arise from the study of these 

 rocks, not the least of which is their thickness. If one assumes them to 

 lie naturally without distortion, layer upon layer, in York and Adams 

 counties, their perpendicular thickness in this region will be not less than 

 sixteen thousand four hundred feet.f The lower bed of this formation, 



* See Note 4, at the end. 



tSee Volume O', 2d G. S. of Pennsylvania, p. 303, by the author. See also by 

 the same "The American New Red Sandstone." Trans. A. I. M. E. ; "The Meso- 

 zoic formation in Virginia," by C. J. Heinrlch; Trans. A I. M. E., Feb., 

 1878; Noles on the Mesozoic of Virginia, by Prof. William M. Fontaine, Am. J. 

 of Sc, January, 1879; and "Some Mesozoic ores," Proceedings American 

 Philosophical Society, April 20, 1877, by the writer. In the article ci'.ed second, 

 and in a review of the others In the American Naturalist for May, L879, I have 

 shown that by calculating the thickness of Prof. II. D. Rogers' Yardleyville 

 section of this formation (First Geological Survey of Pennsylvania) by th9 

 ordinary method, the thickness of beds would appear to be 51,500 feet, or niqe 

 and three-quarter miles. 



