202 Proceedings of the Boijal Physical Society. 



equivalent volume of calcareous matter, instead of being 

 deposited within a space formerly vacant. This fact of 

 replacement is, again, of interest in its bearings upon the 

 origin of eruptive rocks, and will be referred to in this connec- 

 tion further on. It is beautifully shown in the geyserites of 

 the Bathgate Hills, as well as in those of other localities in 

 Scotland. 



II. Minerals of Hypogene Oeigin. 



These have been formed chiefly by hydrothermal agencies, 

 generally at a considerable depth below the surface; and 

 their subsequent exposure is due in all cases to the combined 

 effects of upheaval and denudation. 



B 1. The Original Minerals of Eruptive Rocks. 



(a) Silica and the Silicates. 

 (h) Some Metallic Compounds. 



(c) Accessory minerals, such as Sphene, Zircon, 

 Orthite, Apatite, etc. 



Section («). These naturally group themselves into (1) 

 Quartz and Tridymite ; (2) the Felspars and the Felspa- 

 thoids; (3) the Amphiboles and the Pyroxenes; (4) Olivine; 

 (5) the Micas. 



So far as their origin is concerned, Sections {a), (IS), and (c) 

 may be considered together ; as, in whatever manner they 

 may have arisen, that mode of origin has been common to 

 them all. 



At the present day it is usually considered that all erup- 

 tive rocks have been formed by actual fusion ; and by many 

 writers they are regarded as arising from subterranean 

 cisterns, where their materials are supposed to have remained 

 in a fluid state from the earliest periods in the Earth's history. 

 Other observers are disposed to regard them as re-melted 

 forms of older eruptive rocks, which had formerly been in a 

 solid state. 



A few years ago it was a common belief among field 

 geologists, even amongst those of wide experieuce, that in 

 many cases eruptive rocks represented simply an extreme 



