432 Scientific Proceedings, Royal Dublin Society. 



allowing, of course, a margin for certain natural conditions, which 

 it would be difficult, if not impossible, to reproduce experimentally. 

 Mixtures of fused silicates, such as those from which artificial 

 glasses, slags, and igneous rocks are formed,' may be regarded as 

 solutions. Such silicate solutions have a strong tendency to form 

 an amorphous glass when cooled rapidly — a feature which appears 

 to be connected with the fact that they may be readily over- 

 cooled, and a rapid approach of the molecules prevented. A glass 

 results, therefore, from the solidification of an over-cooled solution. 

 All kinds of glass may be devitrified — that is to say, may be con- 

 verted into a crystalline aggregate. The process consists in main- 

 taining the glass for a long time at a temperature sufficiently high 

 to render it plastic without actual fusion. Now, if an over-cooled 

 silicate solution, capable of producing crystalline bodies, solidifies 

 as a glass, owing to the resistance the viscosity of the mass offers 

 to rapid crystallization, it cannot be said to possess molecular 

 stability. It may contain porphyritic crystals ; but the capability 

 of producing crystalline material will still remain an inherent 

 quality of the vitreous matrix or base. Perpetual strain will be 

 present — in short, there will be an effort on the part of the glass 

 to come to a condition of molecular equilibrium. This, aided by 

 other causes, must eventually conduce to some rude sort of 

 crystallization. The tendency of a stable body like a porphyritic 

 crystal of felspar will be to attract to itself particles of similar 

 constitution, and to surround itself with a fringe of secondary 

 material. It will grow at the expense of the vitreous matrix. 

 Now, in the rocks before us there is no reason why they should 

 not be supposed to. have originally been vitreous in character. 

 There is certainly no conclusive evidence such as that of perlitic 

 structure. Fluidal structure is only slightly apparent ; but they 

 have the structure known as petrosiliceous. 



Professor Bonney, 1 a high authority upon questions of this 

 nature, considers that " we may safely affirm that the majority of 

 the petrosiliceous rocks owe their structure to a peculiar form of 

 subsequent devitrification, and so, as altered rhyolites, obsidiaus, 

 and pitchstones, belong more properly to the metamorphic rocks 

 (of igneous origin.) " Reference should be made to his suggestive 



1 Presidential Address to the Lon. Geol. Soc, vol. xli., 1885, p. 95. 



