82 



urinations will rectify these and many similar 

 blemishes; under the present uncertainty, the 

 most probable places for the doubtful rocks have 

 been chosen ; since no proofs of geological dis- 

 tinctness have been brought forward so as to claim 

 for them decided' places in an arrangement which 

 is intended to be geological. 



Another practice has here also been adopted, 

 which it is requisite to explain in this place. The 

 number of the minerals which constitute the great 

 masses of mixed rocks, appears to be very limited, 

 as will be shown in an appended list. But many 

 other minerals are occasionally found, as is there 

 mentioned, intermixed in rocks, so as to consti- 

 tute remarkable varieties. In most cases, these 

 varieties seem to be of limited occurrence ; the 

 rocks in which they are found, however distinct 

 in specimens, forming portions only, more or less 

 extensive, of the larger and more ordinary 

 masses. 



It is also well known that these accidental mi- 

 nerals often occur, occasionally, or dispersed ly, 

 in rocks, in proportions so small as not to modify 



