482 OVERLYING ROCKS. 



basalts ; and as il is not found in hornblende or 

 augit, it is necessarily derived from the other 

 constituents of these, the compact felspar, or the 

 clinkstone, in which latter ft has also been shown 

 to exist. In repeating these experiments on 

 some of these rocks which appear to contain 

 compact felspar, they have sometimes been found 

 to yield soda only, at others, both that alkali and 

 potash, like the compact felspar of the primary 

 strata. It is not impossible therefore, that there 

 may be two distinct substances confounded under 

 the term compact felspar ; unless the existence of 

 soda alone is, in these cases, held sufficient to 

 prove the presence of clinkstone, and to form the 

 real distinction between this rock and compact 

 felspar. If any alteration in this respect shall be 

 required, it must be reserved till a more extensive 

 analysis of these rocks shall be instituted ; and a 

 more complete arrangement may then be made, 

 to supersede that which is here attempted. 



These simple rocks, therefore, must be con- 

 sidered as those which are peculiarly essential to 

 the members of the trap family ; as it will here- 

 after be seen that there are not many in which 



