42 DESCRIPTION OF THE PRIMARY [Ch. IV. 



ever, that they are intimate mixtures of one or more of 

 these minerals. 



Several instances will be adduced hereafter, which show 

 that mica, shorl, hornblende, and these analogous minerals, do 

 graduate into each other ; and as this is a subject of great 

 importance, and is indispensable to a right understanding of 

 the primary rocks, it ought to be particularly impressed on 

 the mind of the student. " To the geologist," observes the 

 Rev. J. J. Coiiybeare, " who seeks the aid of mineralogy and 

 chemistry, examples of this intimate penetration of one 

 simple mineral by another (so as, in many cases, to alter very 

 considerably the external and empirical characters of that 

 which yet predominates), must be familiar. Many subordinate 

 beds of the earlier greenstone formation exhibit every stage 

 of a similar phenomenon; and an accurate examination would 

 probably show, that most of the substances named petrosilex, 

 corneene, saussurite, jade, and even flinty slate, are, in fact, 

 only admixtures of this nature, in which felspar, varying from 

 its more compact and semitransparent, to its earthy and 

 granular form, is uniformly and intimately penetrated by 

 some variety or other of hornblende, of diallage, and occa- 

 sionally, perhaps, of other minerals, which (as hypersthene) 

 enter more sparingly into the composition of rock masses. 

 Such admixtures can be properly studied only in those endless 

 suites of specimens which Nature herself preserves, and pre- 

 sents in situ. The subject is an interesting one, and well 

 deserves closer attention and investigation than it has yet 

 met with." * 



It is a very common opinion among geologists, that the 

 almost endless varieties of primary rocks, produced by these 

 mineral transitions, embarrass the investigation of this de- 

 partment of the science, and set classification utterly at 

 defiance. But a more enlarged knowledge of this subject 

 will probably prove, that this frequent passage of one rock 



* Annals of Philosophy. New Series, vol. vi. 



