XX 11 PREFACE. 



in the following- work, are, with their justifica- 

 tion, stated whenever they occur. 



It must also be remarked, that although there 

 is a considerable deficiency of the terms which 

 would be required to distinguish the rocks in the 

 following catalogues, according to their mineral 

 characters, the nature of the arrangement here 

 adopted is such as to render this a trifling incon- 

 venience. That arrangement is founded on geo- 

 logical principles ; and, with a few modifications, 

 the general terms already in use, have been found 

 sufficient for the purposes in view. They will 

 at least suffice in the present state of our know- 

 ledge. Should any serious imperfections still 

 exist, or hereafter arise, it will be more prudent, 

 as in some instances has here been done, to make 

 the requisite changes or additions in a gradual 

 manner ; to repair the antient structure rather 

 than to erect a new edifice. Authority of mode- 

 rate influence will thus receive attention when 

 good reasons are superadcled to its weight ; and 

 time will thus perfect a system of terms which, 



