chap. vi. Trachyte mid Basalt. 139 



extended formations of granite and the allied meta- 

 morphic rocks. I have never examined a district in 

 an extensive granitic region without discovering dikes ; 

 I may instance the numerous trap-dikes, in several 

 districts of Brazil, Chile, and Australia, and at the Cape 

 of Good Hope : many dikes likewise occur in the great 

 granitic tracts of India, in the north of Europe, and in 

 other, countries. Whence, then, has the greenstone 

 and basalt, forming these dykes, come ? Are we to 

 suppose, like some of the elder geologists, that a zone 

 of trap is uniformly spread out beneath the granitic 

 series, which composes, as far as we know, the founda- 

 tions of the earth's crust ? Is it not more probable, that 

 these dikes have been formed by fissures penetrating 

 into partially cooled rocks of the granitic and meta- 

 morphic series, and by their more fluid parts, consisting 

 chiefly of hornblende, oozing out, and being sucked 

 into such fissures ? At Bahia, in Brazil, in a district 

 composed of gneiss and primitive greenstone, I saw 

 many dikes, of a dark augitic (for one crystal certainly 

 was of this mineral) or hornblendic rock, which, as 

 several appearances clearly proved, either had been 

 formed before the surrounding mass had become solid, 

 or had together with it been afterwards thoroughly 

 softened. 1 On both sides of one of these dikes, the 

 gneiss was penetrated, to the distance of several yards, 

 by numerous, curvilinear threads or streaks of dark 

 matter, which resembled in form clouds of the class 

 called cirrhi-comas ; some few of these threads could be 

 traced to their junction with the dike. When examin- 



1 Portions of these dikes have been broken off, and are now sur- 

 rounded by the primary rocks, with their laminse conformably wind- 

 ing round them. Dr. Hubbard, also (' Silliman's Journal,' vol. xxxiv. 

 p. 119), has described an interlacement of trap-veins in the granite 

 of the White Mountains, which he thinks must have been formed 

 when both rocks were soft. 



