VOLCANIC KOCKS. 597 



it resembles that of the trap rocks, as will shortly 

 appear; and the imbedded minerals which are 

 peculiar to some volcanic productions, will, as 

 on former occasions, be enumerated in the Synop- 

 tic catalogue. Respecting these, there have been 

 many doubts ; namely, whether they have been 

 ejected in their present forms, whether they are 

 the result of peculiar chemical affinities exerted 

 in the fused mass, and, lastly, whether many are 

 not the produce of subsequent watery infiltration. 

 Many prejudices, also, appear still to exist on 

 this part of the subject ; but the difficulties, such 

 as they are, will not be removed until the sub- 

 stances themselves are better understood. 



With respect to the form of the Synopsis, the 

 author has laboured under a difficulty from 

 which, probably, no one is yet exempt, namely, 

 the want of access to a numerous and authentic 

 collection of specimens. It is probable, that, with 

 such a collection, which does not appear to have 

 been yet completed by any one in a satisfactory 

 manner, the whole might have been thrown into 

 the same form as the synopsis of the trap family. 



