THE CAMBRIAN PERIOD. 247 



shores of the time; there are sandstones, the sand of which was de- 

 posited in shallow water where the waves were sufficiently vigorous 

 to keep the mud from settling; shales representing the deposits made 

 in stiller or deeper water; and beds of limestone representing, for the 

 most part, the accumulations of shells, etc., in regions to which ter- 

 rigenous sediments were not carried in quantity. On the whole, the 

 proportion of limestone is greater in the upper part of the system than 

 in the lower, and toward the south and southeast, rather than in other 

 regions. 



Geographic variations in the sediments. — The distribution of these 

 various sorts of sedimentary rocks, shows that various kinds of detrital 

 beds were accumulating in different places at the same time, and at 

 the same place at different times. Thus in the vicinity of Quebec, 

 the Cambrian system is represented by 5000 to 6000 feet of shale. 

 Between the Adirondack and the Green mountains there are 3000 to 

 5000 feet of shale overlying 1000 feet of limestone, and some of the 

 limestone appears to be the time-equivalent of some of the shale of 

 Quebec. In Washington County, N. Y., the Cambrian strata are 

 10,000 or 12,000 feet thick, the upper and greater portion being of 

 slate (formerly shale) and sandstone, while the lower and thinner part 

 (1400 feet) is limestone. Farther south, the system is composed chiefly 

 of shale (or slate) and sandstone (or quartzite). In Tennessee and 

 North Carolina there are great thicknesses of quartzite overlain by 

 shales. In Georgia and Alabama, the thickness of the system is 

 something like 10,000 feet, the strata being made up of shale and 

 sandstone with some limestone. 



In the southern part of the Appalachian region, the general order 

 of deposition was sandstone, followed by shale, and that by a small 

 amount of limestone. In Virginia, where the system is 3400 feet thick, 

 it is chiefly made up of shale, which rims down well toward its base. 

 North of Virginia, as far as New York and Vermont, there seems to 

 have been a great variety of sediment, including many beds of limestone 

 near the base of the system. Since throughout most of the Appalachian 

 belt, all divisions of the Cambrian are represented, it is evident from 

 the above statements that while limestone was forming in one region, 

 sandstone and shale were forming in others, and vice versa. If the 

 sequence of beds in the Appalachian region be compared with that in 

 other regions, they are found to vary widely. Thus the Upper Cambrian 



