DESCRIPTIONS OF THE FORMATION'S. 39 



beneath the coal-measures. It is a heavy body of limestones and shale, the latter 

 almost one-fourth of the mass ; and there is also a sandstone. See the above 

 description of 13 a. in Illinois. 



In Middle Pennsylvania, around the Broad Top coal-basin, Prof. J. P. Lesley 

 says there appears, for the first time in this formation, going west, distinct traces 

 of the great mountain limestone formation, which underlies all the southern and 

 western coal-fields, and becomes one of the principal features of the geology of 

 the Rocky Mountains, as it is also of the geology of Europe. The red shale 

 formation is here seen, divided hi two 910 feet of it above, and 141 feet of it 

 below ; a middle group of red and gray, mottled calcareous shales, and thin lime- 

 stone layers, full of fossil shells in all 49 feet thick separating the upper and 

 lower members of nearly pure red shale. 



The narrow red shale valleys, which surround this Broad Top coal-basin, the 

 Cumberland basin in Maryland, and the three principal groups of anthracite basins 

 in Eastern Pennsylvania, are due to the thickness and softness of this important 

 formation. But while it is 3,000 feet thick at Pottsville, it is but 300 feet thick 

 along the Allegheny Mountain, and less than 100 feet thick around the coal-basins 

 of Tioga and Bradford counties ; and, therefore, instead of making valleys, only 

 marks the top of the mountain steep slopes with a narrow terrace, over which 

 dominates the vertical cliffs o the outcrop of the coal conglomerate. 



14 a. Millstone Grit. This is a mass of white or yellow sandstone, containing 

 vast numbers of quartz pebbles, and forming a pudding-stone, or conglomerate. 

 It is called the Millstone Grit, from being used for the manufacture of millstones. 

 In Pennsylvania and Virginia the formation is 1,000 feet thick, but becomes 

 reduced to from 10 to 175 feet in Ohio. In Kentucky it is from 50 to 500, and in 

 Indiana from 50 to 100 feet. It is a very peculiar rock, and very wide spread, 

 extending out beyond the coal measures proper, of which it is the base and support. 

 There is not in the entire geological series, says Dr. Newberry, another stratum of 

 rock so widely distributed, and presenting as strongly marked lithological characters, 

 as this. The pebbles are generally of quartz, and well rounded. The sand, which 

 forms the paste, and holds together the pebbles of the conglomerate, is generally 

 coarse, and consists of rounded grains of quartz, which differ from the pebbles 

 only in size. In the anthracite region of Pennsylvania, conglomerate rocks some- 

 times occur between coal-beds, but in the other coal regions they are below all the 

 workable coal-beds. Any cases of thin beds of good coal being found in or below 

 the conglomerate, are exceptional and rare. It does not always maintain its 

 character as a conglomerate, being sometimes an ordinary sandstone. The great 

 lead mines of Joplin and Granby, in Missouri, are in a ferruginous sandstone, the 

 equivalent of the Millstone Grit, or the Chester group, and the Hot Springs of 

 Arkansas are in the Millstone Grit, greatly metamorphosed. 



14 b. and c, Lower and Upper Coal Measures. The series of rock-strata, 

 among which the carboniferous coal-beds are found, are called the Coal Measures, 

 which produce all the best coal of America. They consist of repeated alternations 

 of exceedingly diversified rocks, of every degree of coarseness, from the smoothest 

 fire-clay to exceedingly rough, silicious conglomerates, including within those 

 extremes a wide variety of coal-shales, or mud-rocks, of almost every color and 

 texture marls, argillaceous sandstones and quartzose grits, also thin bands of 

 limestones, both pure and rnagnesian, and numerous seams of carbonate of iron. 



