chap. vn. Volcanic Rocks. 155 



character, and contains much hypersthene. These moun- 

 tains, in their lower half, are generally encased by strata 

 containing numerous small corals and some shells. 

 These shells have been examined by Mr. G. B. Sowerby, 

 and are described in the Appendix : they consist of two 

 species of Producta, and of six of Spirifera ; two of 

 these, namely, P. rugata and S. rotundata, resemble, 

 as far as their imperfect condition allows of comparison, 

 British mountain-limestone shells. Mr. Lonsdale has 

 had the kindness to examine the corals ; they consist 

 of six un described species, belonging to three genera. 

 Species of these genera occur in the Silurian, Devonian, 

 and Carboniferous strata of Europe. Mr. Lonsdale 

 remarks, that all these fossils have undoubtedly a 

 Palaeozoic character, and that probably they correspond 

 in age to a division of the system above the Silurian 

 formations. 



The strata containing these remains are singular 

 from the extreme variability of their mineralogical 

 composition. Every intermediate form is present, 

 between flinty-slate, clay-slate passing into gray-wacke, 

 pure limestone, sandstone, and porcellanic rock; and 

 some of the beds can only be described as composed 

 of a siliceo-calcareo-clayslate. The formation, as far 

 as I could judge, is at least a thousand feet in thick- 

 ness : the upper few hundred feet usually consist of a 

 siliceous sandstone, containing pebbles and no organic 

 remains ; the inferior strata, of which a pale flinty slate 

 is perhaps the most abundant, are the most variable ; 

 and these chiefly abound with the remains. Between 

 two beds of hard crystalline limestone, near Newtown, 

 a layer of white soft calcareous matter is quarried, and 

 is used for whitewashing houses. From information 

 given to me by Mr. Erankland, the Surveyor-General, 

 it appears that this Palaeozoic formation is found in 



