130 TOWN GEOLOGY. [vi. 



us, that we must dig deeper than we have dug as yet, 

 before we reach the graves of the first living things. 



Let this suffice at present for the Cambrian and 

 Laurentian rocks. 



The Silurian rocks, lower and upper, which in 

 these islands have their chief development in Wales, 

 and which are nearly thirty-eight thousand feet thick ; 

 and the Devonian or Old Red sandstone beds, which 

 in the Fans of Brecon and Carmarthenshire attain a 

 thickness of ten thousand feet, must be passed through 

 in an upward direction before we reach the bottom of 

 that Carboniferous Limestone of which I spoke in my 

 last paper. We thus find on the Cambrian rocks 

 forty-five thousand feet at least of newer rocks, in 

 several cases lying unconformably 011 each other, 

 showing thereby that the lower beds had been up- 

 heaved, and their edges worn off on a sea-shore, ere 

 the upper were laid down on them; and throughout 

 this vast thickness of rocks, the remains of hundreds 

 of forms of animals, corals, shells, fish, older forms 

 dying out in the newer rocks, and new ones taking 

 their places in a steady succession of ever-varying 

 forms, till those in the upper beds have become 

 unlike those in the lower, and all are from the 

 beginning more or less unlike any existing now on 

 earth. Whole families, indeed, disappear entirely,- like 

 the Trilobites, which seem to have swarmed in the 

 Silurian seas, holding the same place there as crabs 

 and shrimps do in our modern seas. They vanish after 

 the period of the coal, and their place is taken by an 

 allied family of Crustaceans, of which only one f ornr 

 (as far as I am aware) lingers now on earth, namely, 

 the " King Crab," or Limulus, of the Indian Seas, a 



.',-; \ 



