20o Effects of Heat modified by Compression. 



fragments, evidently detached from masses similar to our 

 common rock, evince the action of some very powerful 

 agent of destruction. Aualogy, too, leads us to believe 

 that all the primary rocks have once been covered with se- 

 condary j yet in vast districts no secondary rock appears* 

 In short, geologists seem to agree in admitting the general 

 position, that very great changes of this kind have taken 

 place in the solid surface of the globe, however much they 

 may differ as to their amount, and as to their causes. 



Dr. Hutton ascribed these -changes to the action, during 

 very long time, of those agents, which at this day continue 

 slowly to corrode the surface of the earth ; frosts, rains, the 

 ordinary floods of rivers, &c, which he conceives to have 

 acted always with the same force, and no more. But to 

 this opinion I could never subscribe, having early adopted 

 that of Saussure, in which he is joined by many of the con- 

 tinental geologists. My conviction was founded upon the 

 inspection of these facts in the neighbourhood of Geneva, 

 which he has adduced in support of his opinion. I was 

 then convinced, and I still believe, that vast torrents, of 

 depth sufficient to overtop our mountains, have swepi along 

 the surface of the earth, excavating valleys, undermining 

 mountains, and carrying away whatever was unable to re- 

 sist such powerful corrosion. If such agents have been 

 at work in the Alps, it is difficult to conceive that our coun- 

 tries should have been spared. I made it therefore my busi- 

 ness to search for traces of similar operations here. I was not 

 long in discovering such in great abundance ; and, with the 

 help of several of my friends, I have traced the indications of 

 vast torrents in this neighbourhood as obvious as those I for 

 merly saw on Saleve and Jura. Since I announced my opi- 

 nion on this subject, in a note subjoined to my paper on 

 vhinstone and lava, published in the fifth volume of the 

 Transactions of this Society, I have met with many con- 

 firmations of these views. The most important of these are 

 derived from the testimony of my friend lord Selkirk, who 

 has lately met with a series of similar facts in North America. 



It would be difiicult to compute the effects of such an 

 agent j but if, by means of it, or of any other cause, the 



whole 



