29 



bably to a certain extent the limestones as well, have not originated 

 from the alteration of either ordinary aqueous sediments, or of volcanic 

 materials. Furthermore, to use Dr. Hunt's words, the " conditions 

 under which these rocks were deposited and crystallized, whether in 

 shallow waters, or in abyssal depths (where pressui-e greatly influences 

 chemical afiinities) have not been rejiroduced to any great extent since 

 the beginning of palaeozoic time," and " the eruptive rocks, or at least a 

 large part of them, are softened and displaced portions of these ancisnt 

 neptunian rocks, of which they retain many of the mineralogical and 

 lithological characters."*' Although, according to Dr. Hunt, the con- 

 ditions under which the crystalline rocks were deposited are not 

 reproduced at present on the earth's surface to any great extent, yet we 

 see the same processes going on in a much less enei-getic form in the 

 action of the thermal waters of Plombieres and other localities in 

 France, on the masonry of the old Roman baths. Various minerals, 

 such as apophyllite, scolezite, opal, etc., have been formed by the action 

 of the heated waters on the constituents of the concrete, and are found 

 filling cavities in the bricks, as well as interstices in the cement. 

 Daubree considers that the lime, alumina and silica of the minerals so 

 produced have been derived from the mortar and the bricks, while the 

 water has chiefly furnished the alkalies. These changes have taken place 

 at temperatures not above 70°. The production of the crystalline 

 rocks at a temperature as low as this does not preclude the possibility 

 of the existence of life on the planet in this early time, indeed Dr. 

 Hunt has argued that in the occurrence of great beds of iron and 

 graphite in the Laurentian, we have evidence of at least an abundant 

 flora. The process supposed by this theory would probably find an 

 analogue in the agencies at work in the formation of mineral veins. 



According to the third hypothesis, which is at present very 

 generally held in England and America, owing largely to the command- 

 ing influence of Zell, though not originated by him, the crystalline i"ocks 

 under consideration may be of various ages, in some cases com paratively 

 recent, but have undergone changes subsequent to their deposition, 

 often metasomatic in character, which have caused them to become 



* The History of some Pre-Cambivan Kocks in America and Europe, p. 3. 

 Bee also, Chemical and Geological Essays by the same author. 



