STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 507 



feet, including partings of ordinary clay. It is finely laminated, but 

 this feature is distinct usually only in " spent clay," that which has 

 been treated. Thin streaks have been discovered in shales within 

 the Carboniferous Limestone, but they are unimportant. Four im- 

 portant horizons are in the Calciferous Sandstone, the chief one 

 being at 3,200 feet below the Limestone base. At some places, the 

 shale has many impressions of fish ; at others it is composed almost 

 wholly of minute cyprids and crustaceans, so abundant that the shale 

 resembles fine linseed cake. With these are fragments of ferns. 

 The lagoon of deposit had an- area of not less than 330 square miles. 

 The best shale has fixed carbon, 5 ; volatile, 25 ; ash, 70 per cent. A 

 yield of 30 gallons of oil per ton is that of good shale. 



The Craigleith Sandstone, at base of the Calciferous, is well 

 marked in the Edinburgh area, whence Witham obtained his tree, 

 which, evidently, was a " snag." Brown^^* described this sandstone 

 as made up of lenses, thinning out in all directions and dovetailing. 

 Coaly laminations, derived from drifted material, are numerous. 

 The water was shallow; sun-cracks, worm tracks, ripple-marks, 

 rainprints and footprints of labyrinthodonts have been observed. 

 Brown found in the quarry a large block of current-bedded sand- 

 stone containing several casts of Lepidodendron. The largest frag- 

 ment, 3 feet long and 14 inches wide, was somewhat compressed and 

 retained some of its bark, converted into coal. At one side in the 

 interior was a thick layer of brown material, but the rest of the 

 cavity was filled with sand. The brown substance contained num- 

 bers of the gasterpod, Platyostomella, and the " nests " were formed 

 before the sand was deposited, for the laminae of the latter curve 

 around them. This gasteropod was probably an estuarine form. 

 At another locality, it is associated with Spirorhis pusillus, which 

 may indicate marine conditions. At the same time, the Craighill 

 species has peculiarities, which lead Brown to suggest that, like 

 Hydrohia, the genus may have had fresh-, brackish- and salt-water 

 species. The question of adjustibility of molluscs to changing ma- 

 rine or fresh-water conditions is unimportant. They can and do 



^1* C. Brown, " On the occurrence of Gasteropods {Platyostomella scoto- 

 hurgalensis) in a Lepidodendron from Craighill Quarry, Edinburgh," Trans. 

 Edinh. Geol. Soc, Vol. VII., 1897, pp. 244-251. 



