340 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



canals of eruption seem to have been developed, which terminate 

 on the surface of the earth in the craters of volcanoes. The tran- 

 sition from the earlier massive forms of deposition to the present pe- 

 culiar volcanic type is so gradual and evident, that it is impossible to 

 ascribe the former to any other cause than that from which the latter 

 has been derived. Moreover it is impossible to discover any 

 lithological difference betweeu the trachytes of many lava streams 

 and other rocks of the same class, which occur constituting whole 

 mountain masses. 



It is further a very remarkable circumstance connected with 

 basaltic intrusions that they have exerted upon the neighbouring 

 strata effects which could only have been produced by great heat. 

 These effects, such as the re-crystallization of limestone, the car- 

 bonizing of coal, etc., are too well known to require particularisa- 

 tion. Another fact which speaks for the igneous origin of basalts 

 is the following : — In many basaltic veins their sides or selvages 

 are composed of a crust of glass or slag, which gradually alters 

 towards the centre of the vein into the granular rock. This 

 circumstance is entirely analogous to that observed in many slags. 

 These are often quite vitreous on the surface where they have 

 cooled quickly, while beneath they assume a granular and even 

 crystalline texture. 



In the first part of this paper I have referred to the chemical 

 composition of certain rocks of the trachytic and basaltic groups. 

 The analyses there given were however of the extremely siliceous 

 trachytes and basic basalts. In Bischof's Chemical and Physical 

 Geology there are recorded 27 analyses of trachytes containing 

 from 52-8 to 72*24 percent, of silica and averaging 62*91 per 

 cent. In the same work there are given 22 analyses of dolerites 

 and basalts, the content of which in silica ranges from 32*5 to 

 52*96 and averages 76*16 per cent. For the sake of completeness 

 I insert here a list of the various species of the tracryte and basalt 

 families as given by Cotta, preparatory to adverting to certain pe- 

 culiarities in the structure of some of them, which peculiarities 

 will again be referred to towards the close of the present chapter 

 in discussing the relation which exists betwixt granite and gneiss. 



Massive Trachytic Rocks. 

 Name. Mineralogical constituents and principal 



characters. 

 Trachyte, Sanidin (glassy felspar) and albite 



with hornblende or mica — granular. 

 * Cotta: Gesteinelehre, p. 78. 



