396 THE CANADIAN NATURALIST. [Oct. 



been percolating for c nturies, and have given rise to various 

 zeolites — apophj'llite and cliabazite among others ; also to calca- 

 reous spar, arragonite, and fluor spar, together with siliceous min- 

 erals, such as opal, — all found in the interspaces of the bricks and 

 mortar, or constituting part of their rearranged materials. The 

 quantity of heat brought into action in this instance in the course 

 of 2000 years has, no doubt, been enormous, although the inten- 

 sity of it developed at any one moment has been always incon- 

 siderable. 



The study, of late years, of the constituent parts of granite has 

 in like manner led to the conclusion that their consolidation has 

 taken place at temperatures far below those formerly supposed to 

 be indispensable. Gustav Rose has pointed out that the quartz 

 of granite has the specific gravity of 2*6, which characterizes silica 

 when it is precipitated from a liquid solvent, and not that inferior 

 density, namely 2*3, which belongs to it when it cools and solidifies 

 in the dry way from a state of fusion. 



But some geologists, when made aware of the intervention on a 

 large scale, of water, in the formation of the component, minerals 

 of the granitic and volcanic rocks, appear of late years to have been 

 too much disposed to dispense with intense heat when accounting 

 for the formation of the crystalline and unstratified rocks. As 

 water in a state of solid combination enters largely into the alumi- 

 nous and some other minerals, and th'/refore plays no small part 

 in the composition of the earth's crust, it follows that, when rocks 

 are melted, water must be present, independently of the supplies 

 of rain-water and sea-water which find their way into the regions 

 of subterranean heat. But the existence of water under great 

 pressure afibrds no argument against our attributing an exces- 

 sively high temperature to the mass with which it is mixed up. 

 Still less does the point to which the melted matter must be cooled 

 down before it consolidates or crystallizes into lava or granite 

 afford any test of the degree of heat which the same matter must 

 have acquired when it was melted and made to form lakes and 

 seas in the interior of the earth's crust. 



The evidence of a period of great cold in England and North 

 America, in the times referred to, is now so universally admitted 

 by geologists, that I shall take it for granted in this Address, and 

 briefly consider what may have been the j^robable causes of the re- 

 frigeration of central Europe at the era in question. One of these 

 causes, first suggested eleven years ago by a celebrated Swiss geo- 



