1819.] Greeuough on the First Principles of Geology. 367 



advantage to. the author himself. The Essay seems to have 

 been written at different times, and with different objects in view, 

 and the subjects are merely placed side by side, or rather one 

 after the other, without sufficient attention to render the bond of 

 connexion evident to the reader. Hence a certain air of obscu- 

 rity in which the opinions of the author are involved ; and I am 

 not sure if he has been always able to avoid inconsistencies and 

 inaccuracies in his reasonings ; but these, if thev happen to 

 exist, will be best pointed out in the analysis of the Essay to 

 which I now proceed. 



The figure of the earth which it actually presents, is called its 

 real figure. Its statical figure is that which it would present if 

 level with the surface of the sea. 



It is well known that the statical figure is an oblate spheroid, 

 and that mathematicians have demonstrated that this is the 

 figure which it would have assumed had it been in a liquid state. 

 The Huttonians, or at least Prof. Playfair, are of opinion that 

 the earth had no beginning;, and, therefore, that it never was 

 liquid, but that it owes its statical figure to attrition and volcanic 

 fire. It appears to me that the inaccuracy of this opinion is 

 capable of being shown by mathematical demonstration. 



The real figure of the earth is known to be uneven on the 

 surface ; in some places rising into mountains ; in others sinking 

 into valleys. These inequalities, when compared with the size 

 of the pdobe, sink into absolute insignificance ; but with regard 

 to man, they are of the highest importance, as his comforts, or 

 probably his very existence, depends upon them ; for if there 

 were no mountains, there would be no rain, and no rivers ; and 

 consequently the earth would be incapable of producing food for 

 the support of its inhabitants. Much discussion has taken place 

 respecting the cause of these inequalities. Our author is of 

 opinion that the valleys have been produced by the removal of 

 matter. It seems to follow as a consequence from this, though, 

 our author no where states it, and indeed in the third essay 

 expressly embraces the contrary doctrine — it seems to follow, I 

 think, that before the valleys were excavated, the surface of the 

 earth was level, or nearly so ; which was the doctrine of Burnet. 



That valleys have been usually formed by the removal of matter, 

 our author considers as established by the following considers.- 

 . : When strata are horizontal, it is usual to find opposite 

 hills of the same elevation composed of the same substances- 

 Of this, various examples are given ; but the fact is so common 

 that it cannot have escaped the notice of the most careless. 

 '",..!-. The strata found on the heights are wanting in the 

 valleys.. But if we follow the beds which occur in the valleys, 

 we had them running under thtf hills, and constituting a part of 

 their substance. It follows from tins, that the valleys did not 

 exist at ur»t, but the strata continued without iuterrupLion till • 



