268 ON THE FORMATION OF ROCKS. (June, 



semblance leaves the nature of their origin to conjecture, 

 I am convinced, that neither my experience, knowledge, 

 nor industry, are adequate to the task of comparing their 

 various differences and resemblances, so as to form an 

 adequate conclusion; but the faults and imperfections in 

 the execution will not, perhaps, injure the propriety of 

 the arrangement or method; for it has always appeared 

 to me necessary to fix some boundary between the know- 

 ledge of facts which must increase with our experience, 

 and the field of conjecture which may, perhaps, on the 

 contrary, diminish as our positive knov\ ledge augments. 

 It is probable that nature has many more ways of ef- 

 fecting the changes, in the form of rocks, than we are 

 acquainted with; and that she employs many agents, the 

 nature and properties of which we are as yet totally ig- 

 norant of; nor is it improbable that she may form the same 

 rock by two or more different agents. When we pretend 

 to limit the operations of nature, to suit our contracted 

 ideas, we most probably do her injustice. To proceed 

 from the known, which we see daily forming, towards 

 the unknown, through a chain of reasoning strictly analo- 

 gous, is perhaps all that our present knowledge will per- 

 mit us to do. 



It is not intended to give a description of all the par- 

 ticular rocks that may constitute a formation, or be sub- 

 ordinate to it, many of them, such as the Topaz -rock, 

 (Avhich has only as yet been found in a bed, in clay slate, 

 forty or fifty feet broad, and from two hundred to three 

 hundred feet long) would tend to confuse: a general de- 

 scription of the formation, with a few observations, is all 

 that I shall attempt. 



