238 SECONDARY LIMESTONES. 



having been formerly considered a transition rock by 

 foreign geologists. The beds are often of great thick- 

 ness, and the series of considerable depth; yet at times, 

 as in Scotland, the reverse ; while they occasionally 

 include thinner strata of clay, shale, and arenaceous 

 rocks, which, in some places, becoming more prevalent, 

 materially affect the calcareous character of this deposit. 

 The limestone itself is sometimes pure, approaching 

 to the crystalline texture ; while, at others, it becomes 

 argillaceous, magnesian, ferruginous, or bituminous, 

 presenting also a great range of colour, through white, 

 yellow, gray, red, and black. It sometimes contains 

 nodules of a cherty flint, disposed like the flints of 

 chalk, and is an occasional repository of lead, copper, 

 zinc, and iron ; including particularly, many lead mines 

 in England. 



The fossil remains belong chiefly to genera different 

 from those of the upper limestone ; rather bearing an 

 affinity to those of the later primary rocks. Fragments 

 of fishes have occurred, together with trilobites ; while 

 corallites and encrinites abound, as the swimming 

 cellular shells are numerous: besides which, many bi- 

 valve and univalve genera, such as orthocera, nautilus, 

 ammonites, melania, turbo, nerita, helix, modiola, 

 rnya, terebratula, cardiurn, and spirifer occur in Eng- 

 land and elsewhere. Fourteen species of plants, con- 

 sisting of equisetums, ferns, lycopodiums, and fuci, 

 have also been observed ; the former indicating that 

 land in which the subsequent coal was produced, as 

 the latter do the joint presence of the sea. In such 

 a locality at least, this limestone was the mud of an 

 aestuary; as has been the case with the lias and others 

 following it. The last feature of this limestone which 

 may be named, is the frequent occurrence of caverns, and 

 of subterranean fissures often giving passage to rivers. 



