84 



which case some of the rejected varieties will na- 

 turally fall into the synopses among those which 

 now exclusively occupy that place. At present, 

 the several minerals which occur in a state of mix- 

 ture, whether more or less extensive, with the 

 ordinary ingredients of any rock, are added at the 

 end of each enumeration ; and, by examining that 

 list, collectors will be enabled to verify any spe- 

 cimen of which the character might, from the 

 prevalence of some adventitious mineral, appear 

 otherwise doubtful. 



It may lastly be remarked on this subject, that 

 the geological connections of some of these mix- 

 tures of essential and unessential minerals, have as 

 yet been so imperfectly described, that, as in the 

 case of the doubtful rocks just mentioned, it is by 

 no means certain that they have been always truly 

 chosen in this arrangement : but the correction of 

 these and many other faults, must be trusted to 

 time, and to an increased attention to a subject 

 nearly in its infancy, 



i. In the present catalogue there will also be 

 foumj some rocks which have scarcely obtained 



