246 Outlines of Geology. 



resign them. It is, at all events, our particular province and 

 duty, to direct bur humble endeavours towards establishing, as far 

 as may be, a strong line of demarcation between facts and opi- 

 nions ; to take care that genuine science, which aims at the inter- 

 pretation of nature, is not checked by the crudities of modern 

 scholastics — that the roses are not smothered by brambles. 



Cornwall has been cited as furnishing upon its coasts favour- 

 able specimens of lowland clay-slate scenery ; its mountainous 

 aspect is seen to great perfection on the western side of Wales, 

 where Snowdon, Plynlimmon, and Cader Idris, with many of their 

 respectable associates, present the peaked summits, the dark and 

 narrow valleys, the terrific precipices, and the fragmented slopes 

 that peculiarly belong to this formation. 



Slate is often traversed by veins rich in the metals, and abund- 

 ant in fine specimens of various crystallized minerals. It is matter 

 of doubt whether it ever contains organic remains, though the 

 impressions of some bivalve shells are said to have been found in 

 it. I am, however, rather inclined to believe that these im- 

 pressions are either in grauwacke slate, or in that which borders 

 upon and passes into limestone, and that genuine clay-slate is 

 destitute of shells — a fact which appears opposed to its imagi- 

 nary aqueous origin, and which has given rise to the presump- 

 tion that the ocean was uninhabited by living beings at the time 

 that this great deposit was produced. 



Not the least-important fact in the history of the rocks which 

 have now been considered, is their gradual, and often insensible, 

 transition, as it were, into each other ; for it furnishes us with a 

 strong argument against many of those speculations into which 

 some geologists have entered respecting their origin and forma- 

 tion. The gradual migration of chalk into lias and sandstone, 

 and of these into oolitic deposits — of oolite into lias, and of this 

 again into limestone, is visible in many hand-specimens, and there 

 are equally well-marked instances of the transition of clay-slate 

 into red sandstone on the one hand, and on the other into mountain 

 limestone and into grauwacke ; so that those rigid lines of demar- 

 cation, and sudden transitions, which the student might be led to 



