28 





may have risen in a state of gas or vapour through the joints. But, without resorting to 

 this hypothesis , let us suppose that , from the first , all the ingredients existed together 

 in the mass. The external portion of the cube etc. differs more in mineralogical than in 

 chemical character from the internal. The schorl of the former, compared with the felspar 

 of the latter, has a great excess of alumina and a great deficiency of silica. But this dif- 

 ference is compensated, and the balance of ingredients restored, by the quartz which ac- 

 companies the schorl. If we therefore reduce the internal and external portions to their 

 constituents, we shall find that the essential difference is only about 10 per cent, and that 

 it consists in the latter having about 9 per cent of oxide of iron instead of only 2 per 

 cent, and in having about 2 per cent of a new ingrediënt, — boracic acid. We can hardly 

 err in attributing the difference mainly to the chemical action of the acid, which, whether 

 by itself or in combination with soda, is remarkable for its fusibility, and its power of com- 

 municating this property to compounds. The tendency of certain substances in a fused mass 

 to retire towards the surface when other substances are there found for which they have a 

 stronger affinity than the other constituents of the mass is well known. But there is no 

 difficulty in conceiving that such a transfer may have been mainly mechanical. The boracic acid 

 may have retained a portion pf the matter with which it was in combination in a fluid 

 or viscid state, for some time subsequent to the crystallization and partial solidification of 

 the felspar etc, and the internal pressure of the semi-solidified mass alone may have 

 forced this towards the sides and caused it to rise to the surface. It is ascertained that 

 both felspar and quartz remain in a viscid state at temparatures greatly inferiour to that 

 at which they are fused, and hence there must have been a degree of internal pressure sub- 

 sequent to crystallization. The fugitive character of schorl is more than once pointed out by Sir 

 H. de ia. Beghe in his Report. Amongst other instances of alterations produced on sedimen- 

 tary rocks by contact with granite he mentions some slates in which schorl has been intro- 

 duced between the laminae. A more remarkable case occurs in a granite consisting of large 

 felspar crystals in a base of schorl and quartz. At some places the felspar crystals have 

 been decomposed and replaced by crystals of schorlcrossing each other in various directions, 

 and the schorl in the surrounding base is evidently deficiënt. 



The abundance of quartz at the surface may be due in some measure to the circumstance 

 that the boracic acid , whether expelled from the interiour on the crystallization of the 

 felspar and mica or derived from without through the joints, would , wherever its ultimate 

 locality was ; be hostile to the formation of felspar and mica there. 



However we may account for it , the fact of the schorl taking the place of felspar and 

 mica in the Devonshire and Cornwall granites, appears to be analogous to that of hom- 

 blende replacing mica in the granites and syenites of Pulo XJbin , and if we extend the 

 preceeding speculations regarding the one transition to the other, we shall find them in har- 

 mony with the views formerly expressed. We must in the first place consider the Island 

 itself as the summit of one granitic bubble , of which much of the external portions have 

 disappeared. A portion of the bubble (its superjacent rocks , whatever they were , having 

 been swept away) is now elevated above the sea , so as to exposé a belt below high water 

 mark to the action of the waves which are working into the nucleus , and , in their pro- 



