167 



also an ingredient in some of the compounds of 

 the family of overlying rocks, to which it appears 

 to be limited. 



COMPACT FELSPAR. 



This mineral, not always very strongly dis- 

 tinguished from the former, is found in a similar 

 manner ; either simple, or entering into a great 

 number of very important and extensive com- 

 pound rocks. It probably possesses some variety 

 of composition which has not vet been ascer- 

 tained. It is further certain that it differs from 

 crystallized, or common felspar, in its chemical 

 nature ; the specimens hitherto examined having 

 been found to contain soda together with potash, 

 when common felspar contains potash alone. 

 It is a subject yet deserving the attention of 

 chemists, to enquire whether, instead of these 

 alkalies, lithion may not enter into the compo- 

 sition of some of the minerals in question. 

 Whenever the true nature of compact felspar 

 shall be decidedly ascertained, it would conduce 

 to perspicuity if it were distinguished by some 

 other title. 



