1818.) ON THE FORMATION OF ROCKS. 267 



ing some parts which have a distant relation to both the 

 modes of formation. 



As nature does not advance by large leaps, but by- 

 small and regular steps, leaving no marks in the chain of 

 gradation on which we can place the limits of our artifi- 

 cial division, the line of demarkation between the first 

 and second classes will be doubtful; and the rocks ap- 

 proximating on both sides, will not be well determined. 

 The line also must change with the progress of our know- 

 ledge and discoveries, and rocks placed in the second 

 class now, because we have not found analogous rocks 

 in the first class, may change their place by new disco- 

 veries, and pass from the second to the first class, or 

 from what may be called the unknown into the known, 

 whenever future experience and observation have thrown 

 light on their origin. 



There is no question here concerning the relative pe- 

 riod in which the different formations by water or fire 

 have originated. This is difficult to ascertain; and fromi 

 the numberless derangements in the original order, liable 

 to many exceptions, nor is the necessity of it evident in 

 the inquiry concerning the origin. Nothing within our 

 observation proves the priority of one mode of formation 

 over the other, nor militates against the probability of one 

 formation often alternating with another, and it is more 

 than probable that the reason we have so few instances of 

 such an alternation on record is because there is so small 

 a proportion of the crust of our globe accurately exa- 

 mined. 



In attempting to separate the rocks, whose origin 

 comes within the sphere of our positive knowledge, or 

 positive analog}% from those whose faint and distant re- 



