Origin of certain Granites, ^c. 57 



the invariable normal form of all such vesicles in lavas, etc., and that 

 the drawn out or elongated flattened form is a subsequent modifica- 

 tion assumed by the spherical vessels, either under the influence of 

 superincumbent pressure, or from the lava cooling more slowly,^ and 

 thus allowing the movement of the mass, when still in a viscid 

 condition, to draw out the spherical vesicles into somewhat of an 

 almond-like shape. Abundant instances, however, could be cited 

 where both lavas and traps also have retained their original spherical 

 shape over extensive areas, probably from their having cooled some- 

 what more quickly, or from having not had much motion communi- 

 cated to them under solidification. 



Again, page 532 of the Geol. Mag., it is stated in a foot-note that 

 " It is certain, however, that rocks such as diallagite, hypersthenite, 

 diorite, syenite, and even granite itself can be developed directly 

 from aqueous rocks, etc." This assertion appears to be put forth 

 solely upon the authority of Mr. James Geikie, for a careful examina- 

 tion of the literature of the subject in the English, French, German, 

 Spanish, Italian, Swedish, and. Danish languages, certainly does 

 not confirm it, and it is to be hoped that geologists will require 

 positive facts, and not vague hypotheses, before they accept such a 

 statement for granted. 



The writer believes that in several cases, at least, where views 

 of this nature have been entertaiaed, they have been formed by 

 persons not much acquainted with petrology or mineralogy, from 

 their confounding certain rocks with names which did not in reality 

 pertain to them. An example, illustrative of this, may be cited : 

 The writer of these remarks, finding from an examination of the 

 sheets of the Geological Survey that large masses of greenstone 

 were represented as occurring in Cornwall, near Penzance, and at 

 the BotaUack mines, immediately imagined that he would there find 

 the same relations of this greenstone to the metallic lodes, occurring 

 as he had found to be the case in South and North America, Spain, 

 Norway, Sweden, etc., and made a journey expressly for this exami- 

 nation ; on arrival he at once found that the rocks had evidently 

 been metamorphosed in situ, and they no doubt originally had only 

 been the ordiaary sedimentary clay-slates. Had he now been content 

 with the decision of the Geological Survey that the rocks in question 

 really were greenstones, then he must at once have come to the con- 

 clusion that greenstones could be formed by the alteration of clay- 

 slates in situ. It did not, however, require a long examination to 

 prove that the rocks were neither petrologically, mineralogically, 

 or chemically, greenstones, or even any allied rock, being nothing 

 more than clay slates altered in situ and possessing none of the pro- 

 perties of greenstones beyond the greenish tinge which coloured 

 them. 



Geologists have been accustomed to define, as eruptive or in- 

 trusive rocks, such rocks as are met with apparently breaking 

 through, protruding into, or sending out ramifications, dykes, or 

 veins, iato the adjacent stratified deposits. 



Mr. James Geikie (p. 530, Gbol. Mag.) refuses any longer to 



