SURROUNDING THE LANd's-END MASS OF GRANITE. 425 



combinations of these secondary products, and to follow the gradual 

 assumption of definite crystalline forms. In the rock from Hose 

 Hill, for example, one specimen contains a quantity of flocculent 

 green matter destitute of any definite character, except in one or 

 two places, where there is evidently an approach to crystallization, 

 whereas in another piece nearly the whole of the green substance 

 exhibits the well-marked characteristics of hornblende. 



It appears, moreover, that two rocks of similar origin and compo- 

 sition may follow different lines of metamorphism, and thus become 

 converted into two widely different substances, the ultimate result 

 depending upon the nature of the active agent and the duration 

 of its operation. The Clicker-Tor dolerite, for example, situated 

 at a distance from the granite, has been converted by aqueous 

 agencies into a serpentinous rock ; while the Penzance dolerites, in 

 closer proximity to the granite, have been transformed into horn- 

 blendic rocks differing from each other in texture and state of altera- 

 tion in accordance with the coarseness or fineness of their original 

 crystallization. 



There remains one important point to be noted. Many of the 

 metamorphic rocks described in the first part of this paper have 

 undergone a second series of changes, in which chemical forces have 

 evidently played the principal part, this subsequent alteration being 

 indicated by the occurrence of micaceous and chloritic pseudomorphs 

 after tourmaline, and an alteration (probably hydration) of the mica. 



The granite has also suffered similar changes. 



The origin of the metamorphic crystalline schists has hitherto been 

 involved in the greatest obscurity ; and it is with the hope of gaining 

 some insight into the nature of the processes concerned in their 

 formation, that I have commenced the inquiry with an attempt to 

 ascertain the nature and extent of metamorphism produced by at 

 least one powerful agent which must necessarily have brought into 

 play enormous chemical and mechanical forces. 



The results already obtained from a very limited number of ob- 

 servations afford sufficient encouragement to continue the investiga- 

 tion ; for it appears not only that clay-slate may be transformed 

 into mica- schist, gneiss, and tourmaline-schist, but that, among the 

 more basic rocks, hornblende-schists may be metamorphosed igneous 

 rocks, some being derived from dolerites or gabbros, while others 

 are very probably foliated diorites. 



In concluding these notes I will refer very briefly to one other 

 point of interest, namely the origin of the granite. In Cornwall, 

 neither examination in the field, nor a microscopical study of the 

 rocks, lends the slightest support to the notion that granite is a 

 metamorphic rock in any proper sense of the term. 



On the contrary, there is the clearest evidence of the former exist- 

 ence of deep-seated volcanic action ; for not only have the slates 

 been subjected to the violent disturbance and alteration previously 

 pointed out, but the surrounding country is seamed for miles by vast 

 numbers of granitic and felsitic dykes (" elvans ") ; and considering 

 the enormous amount of denudation to which the entire district has 



