412 SKETCH TOWARDS A 



could say, " I never write any thing but truth, and 

 never publish anything to others which I am not fully 

 persuaded of myself and do not think that I understand. 

 So that I never have need of false colours to set off 

 the weak parts of an hypothesis, or of obscure expres- 

 sions or the assistance of artificial jargon to cover an 

 error of my system or party." 



Before preceding to this Sketch, however, it is neces- 

 sary tostatethose demonstratedfacts in geology to which 

 a reference must be frequently made or implied, as to 

 fundamental principles; that I may command the ne- 

 cessary brevity. I have called them demonstrated facts, 

 because I believe them such, notwithstanding the con- 

 troversies to which many may still give rise, among 

 the advocates of antient hypotheses; and I believe them 

 such, on the evidences which I have given in the pre- 

 ceding work, as, to that I must refer; since a recapi- 

 tulation of such proofs is here impracticable. It will 

 be for the reader to judge of their value; while I trust 

 that he, like myself, will turn a deaf ear to all that is 

 opinion and assertion, to all that is not Evidence, and 

 to all authority but the authority of Nature herself. 



Ignited fluids, formed of certain earths, become 

 rocks by cooling; and these, not disposed in strata, 

 but in irregular masses and veins, are of various aspects 

 and qualities, presenting also different crystalline and 

 concretionary structures, which are determined, partly 

 by the original materials, and partly by the modes and 

 times of cooling. 



From an identity of circumstances pervadingvolcanic 

 rocks and the several unstratified ones, granite, por- 

 phyry, trap, and others, the same origin is inferred of all ; 

 and it is also proved that such igneous rocks have been 

 produced extensively, at various and very distant periods. 

 The chemical powers of air and water change the 



