49 



which it implies, renders the investigation more 

 easy to the student. It is also justified by the 

 practice of botanists, in pointing out, under the 

 genera to which they rigidly belong in one sense, 

 those species which, from other characters, it is 

 expedient to place under a different genus. 



It is probable that many rocks are omitted, 

 but the arrangement admits of perpetual correc- 

 tion. It was only in my power to describe accu- 

 rately those to which I have had access ; and they 

 are chiefly therefore from Britain, as complete 

 collections of rocks have not as yet been formed 

 by geologists, and few foreign specimens are to 

 be found in the cabinets of mineralogists in this 

 country. It is probable, however, that the enu- 

 meration contains the far predominant number of 

 rocks known ; as the same substances have been 

 found to occur all over the world. No imaginary 

 varieties are given ; a practice in which it would 

 be very easy to indulge, as other authors have 

 <Jone. Such varieties may readily be conceived 

 and multiplied ; but to adopt them is to abandon 

 the study of nature. 







