PALEOZOIC HOCKS AND ERA. 



321 



which these remains were entombed. Imagine, then, a 

 large sigillaria tree on the borders of a coal-swamp, rotted 

 down to a hollow stump. A flood then carried thither 

 floating insects, shells, and carcasses of amphibians, which 

 lodged in the hollow stump, and were covered up with 

 sand. The hollow stump 

 changed to coal, the sand to 

 sandstone, and the animal re- 

 mains to fossils. 



The very earliest amphibian 

 known is recognized by its 

 tracks (Fig. 255). These are 

 found in the sub-Carboniferous of Pennsylvania, in a sand- 

 stone marked with ripple-marks. The animal has been 

 called Sauropus primcevus (primeval reptile-foot). It 

 was evidently a large Labyrinthodont. Not only tracks 

 and ripple-marks, but also rain-prints (Fig. 256) and sun- 

 cracks, are common in the coal-measures. 



FIG. 254. Section of hollow sigil- 

 laria stump, filled with sandstone. 

 (After Dawson.) 



FIG. 5J55. Slab of sandstone with sun-cracks a, and reptilian footprints >, from 

 coal-measures of Pennsylvania ; x $. 



Some General Observations on the Whole Paleo- 

 zoic. Before leaving this long and diversified era, we 

 must look back and make some general observations. 



Progressive Change. During the whole time we 

 may observe a progressive change going on : 1. There 

 was, as we have seen, a steady growth of the continent 



LE CONTE, GEOL. 21 



