OF THE EARTH. 463 



begins. But it forms only a portion of what belongs 

 to a Theory of the Earth. It is an attempt to detail 

 and establish but one train of facts, yet of important 

 ones, and to indicate the nature of the evidence and 

 the reasoning by which they are supported. 



On the Revolutions concerned in the primary Strata. 



I have formerly shown, that whatever differences of 

 antiquity there may be among rocks, we can only 

 prove those relative periods, satisfactorily, in the stra- 

 tified ones, under sonic exceptions which I need not 

 now repeat ; and to these therefore must this inquiry 

 be confined. Whatever relations these may bear to 

 the horizontal plane, their geological inferiority or 

 superiority is unaffected by those, and is assignable : 

 and it is in the lowest, therefore, under this sense, 

 that we must seek the first or original rocks : not the 

 " primordial" ones of theorists, but the first respecting 

 which we can procure unquestionable evidence. And 

 we know that they are prior to any nnstratified rock, 

 because it is the production of these which has changed 

 the positions of the others. The latter, taking 

 them as one mass, form the primary strata of the 

 present arrangement of geologists, as the lowest of 

 this set is the oldest stratum ; while the consecutive 

 general parallelism of the whole, proves that they have 

 been formed during one interval of repose in that ocean 

 under which their materials were accumulated. 



If now there were no other evidence than that of 

 position, for the antiquity of a rock, these primary 

 strata, generally considered, would be the most antient ; 

 and the lowest of them, supposing this ascertained, the 

 first of all. But I have shown that there is another 

 kind of evidence, to be derived from their structure, 

 or ingredients; and, by reasoning from this, we can 



