128 TOWN GEOLOGY. [vi. 



probable by the simple fact that it can be turned into 

 inud again. If you grind up slate, and then analyse 

 it, you will find its mineral constituents to be exactly 

 those of a fine, rich, and tenacious clay. The slate 

 districts (at least in Snowdon) carry such a rich clay 

 on them, wherever it is not masked by the ruins of 

 other rocks. At Ilfracombe, in North Devon, the 

 passage from slate below to clay above, may be clearly 

 seen. Wherever the top of the slate beds, and the 

 soil upon it, is laid bare, the black layers of slate may 

 be seen gradually melting if I may use the word 

 under the influence of rain and frost, into a rich 

 tenacious clay, which is now not black, like its parent 

 slate, but red, from the oxidation of the iron which it 

 contains. 



But, granting this, how did the first change take 

 place ? 



It must be allowed, at starting, that time enough 

 has elapsed, and events enough have happened, since 

 our supposed mud began first to become slate, to allow 

 of many and strange transformations. For these slates 

 are found in the oldest beds of rocks, save one series, 

 in the known world ; and it is notorious that the older 

 and lower the beds in which the slates are found, the 

 better, that is, the more perfectly elaborate, is the 

 slate. The best slates of Snowdon I must confine 

 myself to the district which I know personally are 

 found in the so-called " Cambrian " beds. Below 

 these beds but one series of beds is as yet known in- 

 the world, called the " Laurentian." They occur, to a 

 thickness of some eighty thousand feet, in Labrador, 

 Canada, and the Adirondack mountains of New York : 

 but their representatives in Europe are, as far as is. 



