PROCEEDINGS OF THE SOCIETY. 723 



fore proposed to limit his remarks to three kinds of rocks — the Sandy, 

 the Muddy, and the Calcareous. To begin quite at the outset, he ought 

 to show them the materials themselves — sand, mud, and pounded shell ; 

 but he proposed to pass over these, and give some illustrations of the 

 rocks formed from them by means of sections of such rocks shown upon 

 the screen. It would have made these sections more conclusive if they 

 had been shown by polarised light, the effect of which might be com- 

 pared to that of a coloured map as contrasted with the same only in 

 outline. This, however, involved so much loss of light, that he had 

 decided not to use it. 



In illustration of the effects of water, a section was shown of in- 

 durated organic sand from the Torres Straits — probably a comparatively 

 modern rock — in which the various materials comprising it were 

 cemented together by very small crystals of calcite ; this was followed 

 by a section of a more ancient limestone rock. A section of sandstone, 

 consisting of grains of quartz, was then exhibited, showing how the 

 material was cemented by the deposition of minutely crystalline quartz, 

 probably brought about by the action of water. Next he exhibited a 

 piece of hardened mud-stone of great age. 



As regarded the effects of pressure, it was pointed out that when a 

 quantity of flakes were pressed together they arranged themselves 

 parallel, and gave rise to cleavage, but if a solid body consisting of 

 fair-sized grains, like a granite, was thus acted upon, it was either 

 crushed or sheared. The solvent power of water was also increased by 

 pressure, so that when felspar was crushed, water partially dissolved it, 

 and when the pressure was removed some of the constituents went back 

 into mineral form, but as it had generally lost some of its alkalies, it 

 then commonly took the form of mica. In illustration of the effects of 

 pressure, a large number of sections were shown, and the special features 

 in each pointed out. These included granite from Wicklow, an un- 

 crushed rock formed of two kinds of felspar and two kinds of mica ; 

 granite from Brittany which had been subjected to greater pressure, and 

 two specimens considerably more crushed ; hard felspathic sandstone 

 from N.W. Scotland, which had been exposed to various degrees of 

 pressure, and quartzite from the same under similar conditions ; slate 

 from the Ardennes, consisting of mud blackened by carbonaceous matter ; 

 and slate from the Isle of Man, exhibiting, secondary, or " strain-slip " 

 cleavage. As illustrating the changes chiefly due to heat, a further 

 series of sections was exhibited, including specimens with Chiastolites, 

 Andalusites, brown mica, &c, from Skiddaw and Brittany. Other sec- 

 tions showed examples of changes produced by heat in calcareous rocks, 

 including two from Montreal, in one of which the remains of fossils 

 could be traced, while from the other they had disappeared in the re- 

 crystallisation of the calcite. 



The series was concluded by the exhibition of a set of three speci- 

 mens of rocks, the past history of which was not at present perfectly 

 clear, the first of these showing the effect of puckering by pressure, and 

 the others exhibiting secondary cleavage structure, with re-crystallisa- 

 tion. Prof. Bonney expressed the hope that he had been able to show, 

 by the action of the three influences referred to, that the nature of rocks 

 might be entirely changed, and that a rock which began from mud 



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