Strata and Fossils. \ 5 



cases after consolidation, they have been so much al- 

 tered by heat and other agents of metamorphism, as to 

 have lost almost all signs of their original stratification, 

 while sometimes they are almost undisturbed, except by 

 mere upheaval above the sea : in other cases the beds 

 have been violently contorted, in the manner shown in 

 diagram No. 3. 



Next comes the question : Under what special con- 

 ditions were given areas of these rocks formed ? 



Some formations, such as great part of the Silurian 

 rocks of Wales and its neighbourhood, consist essentially 

 of deposits that were originally marine mud and sand, 

 accumulated bed upon bed, intercalated here and there 

 with strata of limestone, the whole being many thou- 

 sands of feet in thickness. These have since been 

 hardened into rock. Others, like the Old Red Sand- 

 stone, were originally spread out in alternating beds of 

 mud, sand, and stony banks, all coloured red by pre- 

 cipitation of peroxide of iron. Others, like the Liassic 

 and Oolitic deposits, were formed of alternating strata 

 of clay, sand, and limestone ; while others, like the 

 greater masses of the Carboniferous Limestone and the 

 Chalk, were formed almost wholly of carbonate of 

 lime. 



When we examine such rocks in detail, we often 

 find that they contain fossils of various kinds shells, 

 corals, sea-urchins, crustaceans, such as crabs and 

 trilobites, the bones, teeth, and scales of fishes, &c., 

 land plants, and more rarely the bones of terrestrial 

 animals. For instance, in the bed of sandstone. No. 4 

 (fig. 1), we might find that there are remains of sea- 

 shells ; occasionally but more rarely similar bodies 

 might occur in the conglomerate, No. 3 ; frequently 

 they might lie between the thin layers of shale in 



