LOWER COAL MEASURES. 83 



the individual layers which follow and replace one another up- 

 ward and laterally in rapid succession. Often within a vertical 

 distance of a few inches or a few feet, lajers of sand, clay or 

 shale are succeeded by different strata, or else are changed 

 both in color and chemical composition. 



The rocks of the upper Carboniferous are principally shales, 

 clays, sandstone, limestone and coal. Of these the clay-shales 

 greatly predominate. The latter are often bituminous, calcare- 

 ous or arenaceous, and as a matter of course pass gradually in 

 places into sandstone, limestone and coal. On exposure to 

 the atmosphere they quickly disintegrate into soft plastic clays, 

 easily carried away by running water. For the most part they 

 are ashen, dark or black in color, though red, yellow, buff and 

 blue shades are not of uncommon occurrence. In many locali- 

 ties variegated shales prevail — blue, red, yellow and ashen 

 indiscriminately mingled. 



The light-colored shales occasionally afford impressions of 

 ferns and lepidodendron roots, but as a rule they are not very 

 fossiliferous. The dark-colored bituminous varieties, on the 

 other hand, are frequently highly charged with organic remains. 



The first of these shales, by the gradual addition of fine 

 sandy material, pass imperceptibly into sandy shales, and these 

 again into shaly sandstones, and finally into hard, compact 

 sandrock. This gradual transition may take place laterally in 

 the same horizon, or vertically from one layer to another. By 

 a constant increase of carbonaceous matter, the dark-colored 

 shales become highly bituminous and then coaly.' 



There is a large amount of sandy material present in the 

 Coal Measures of the State, but it is usually mixed with clay 

 to such an extent as actually to form sandy shales. In some 

 cases, however, the sand constitutes a rock sufficiently com- 

 pact to afford material for ordinary rough masonry. The sand- 

 stone is usually light-colored and more or less friable. In the 

 direction of the seaward dip the sandstones commonly pass by 

 nearly imperceptible degrees into sandy shales and finally into 

 clay shales — the individual layers becoming rapidly thinner and 

 thinner, usually as the proportions of clay increase. The beds 



