PART L CHAPTER IX. 



123 



Passage of Granite into Trap Granite Veins, 



cumstances, form very different rocks. The same lava, for 

 example, may be glassy, or scoriaceous, or stony, or porphyritic, 

 according to the more or less rapid rate at which it cools ; and 

 some trachytes and syenitic-greenstones may doubtless form gra- 

 nite and syenite, if the crystallization take place slowly. 



It would be easy to multiply examples and authorities to prove 

 the gradation of the granitic into the trap rocks. On the western 

 side of the fiord of Christiania, in Norway, there is a large dis- 

 trict of trap, chiefly greenstone-porphyry, and syenitic-green- 

 stone, resting on fossiliferous strata. To this, on its southern 

 limit, succeeds a region equally extensive of syenite, the passage 

 from the volcanic to the plutonic rock being so gradual that it is 

 impossible to draw a line of demarcation between them. 



" The ordinary granite of Aberdeenshire," says Dr. MacCul- 

 loch, " is the usual ternary compound of quartz, felspar, and 

 mica ; but sometimes hornblende is substituted for the mica. But 

 in many places a variety occurs which is composed simply of 

 felspar and hornblende ; and in examining more minutely this 

 duplicate compound, it is observed in some places to assume a 

 fine grain, and at length to become undistinguishable from the 

 greenstones of the trap family. It also passes in the same unin- 

 terrupted manner into a basalt, and at length into a soft clay- 

 stone, with a schistose tendency on exposure, in no respect 

 differing from those of the trap islands of the western coast.'"* 

 The same author mentions, that in Shetland, a granite composed 

 of hornblende, mica, felspar, and quartz, graduates in an equally- 

 perfect manner into basalt. f 



In Hungary there are varieties of trachyte, which, geologi- 

 cally speaking, are of modern origin, in which crystals, not only 

 of mica, but of quartz, are common, together with felspar and 

 hornblende. It is easy to conceive how such volcanic masses 

 may, at a certain depth from the surface, pass downwards into 

 granite. 



I have already hinted at the close analogy in the forms of cer- 

 tain granitic and trappean veins ; and it will be found that strata 

 penetrated by plutonic rocks have suffered changes very similar 

 to those exhibited near the contact of volcanic dikes. Thus, in 

 Glen Tilt, in Scotland, alternating strata of limestone and argil- 

 laceous schist come in contact with a mass of granite. The 

 contact does not take place as might have been looked for, if the 

 granite had been formed there before the strata were deposited, 

 in which case, the section would have appeared as in Fig. 111.; 

 but the union is as represented in Fig. 112., the undulating out- 

 line of the granite intersecting different strata, and occasionally 



* Syst. of Geo!., vol. i. p. 157. 



t Ibid., p. 158. 



