Fig. 387.
Series of reptilian footprints in the coal-strata of Westmoreland County, Pennsylvania.
a. Mark of nail?
The geological position of the sandstone of Greensburg is perfectly clear, being situated in the midst of the Appalachian coal-field, having the main bed of coal, called the Pittsburg seam, above mentioned ([p. 331.]), 3 yards thick, 100 feet above it, and worked in the neighbourhood, with several other seams of coal at lower levels. The impressions of Lepidodendron, Sigillaria, Stigmaria, and other characteristic carboniferous plants, are found both above and below the level of the reptilian footsteps.
Analogous footprints of a large reptile of still older date have since been found (1849), by Mr. Isaac Lea, in the lowest beds of the coal formation at Pottsville, near Philadelphia, so that we may now be said to have the footmarks of two reptilians of the coal period, and the skeletons of four.[340-A]
CARBONIFEROUS OR MOUNTAIN LIMESTONE.
We have already seen that this rock lies sometimes entirely beneath the coal-measures, while, in other districts, it alternates with the shales and sandstone of the coal. In both cases it is destitute of land plants, and usually charged with corals, which are often of large size; and several species belong to the lamelliferous class of Lamarck, which enter largely into the structure of coral reefs now growing. There are also a great number of Crinoidea (see [fig. 388.]), and a few Echinoderms, associated with the zoophytes above mentioned. The Brachiopoda constitute a large proportion of the Mollusca, many species being referable to two extinct genera, Spirifer (or Spirifera) ([fig. 389.]), and Productus (Leptæna) ([fig. 390.)].
Fig. 388.