160 



schist, when the felspar is white so as to be mis- 

 taken for quartz. 



\ 



Circumstances which have neither constancy 

 nor precision, can have no value as distinctive 

 characters. The colours of rocks, therefore, 

 must only be considered as adventitious and 

 variable features. But, as constituting a portion 

 of the natural history of these substances, and 

 being often very conspicuous and ornamental, 

 they deserve attention. This is more particu- 

 larly the case where, as in marble, rocks are 

 used for the purposes of ornament, and where 

 the very species (in an (economical sense) are con- 

 stituted by modifications of colour. 



Hence, in the following catalogue of rocks, 

 the colours are enumerated among those inferior 

 distinctions which do not aspire even to the 

 rank of varieties. But as language has no terms 

 by which to convey ideas of the innumerable 

 tints by which 'the pure colours are modified, 

 this enumeration has been made in the most 

 general manner ; and, it may be added, with 

 very little anxiety to render it complete. It is 



