218 Proceedings of the Royal Physical Society. 



solves itself into a consideration of the processes by which the 

 enigmatical rocks known as Pegmatites have originated. 

 I have elsewhere speculated upon one or more possible 

 means by which these pegmatites may have been formed. 

 It is true that these speculations were based chiefly upon 

 an indoor study of specimens which are in the Collection 

 of the Geological Survey, and in the still finer series in the 

 Scottish Mineral Collection ; but I have also made the most 

 of several opportunities of studying these pegmatites in the 

 field. The sequence of causes to which these appear to me 

 to be due is as follows : — First, a slow crushing, perhaps 

 repeated more than once. Granulitic structure follows as a 

 consequence of the later movements. This structure reduces 

 the rock material to a condition in which its constituents 

 present a relatively large surface area, which thereby greatly 

 facilitates subsequent, chemical change. The energy of motion 

 arising from the earth movements may generate sufficient heat 

 to anneal the granulitic compound, bat even in the cases where 

 the temperature is not sufficiently high to produce that result, 

 it may tend, especially if alkaline waters (from the bed of 

 the sea or other sources) be present, to bring about fusion. 

 The pressure raises the fusing-point, and counteracts this 

 tendency to pass into the fluid state. But local relief of 

 pressure now and then ensues, whereupon the potentially 

 fluid mass is placed under conditions which favour molecular 

 rearrangement. The process of change is usually a slow 

 one, equally towards the fluid condition and towards the 

 solid condition which afterwards ensues. Hence coarsely 

 crystalline structures are developed, and types of com- 

 binations of crystals (graphic, micrographic, and granitic 

 structures), which can only arise under plutonic conditions 

 and under pressures which, although always great, yet vary 

 in amount in different cases. 



I am inclined to lay special stress upon the probability of 

 these processes having usually been of extreme slowness. 

 If that was really the case, there does not seem any valid 

 objection to the view that, with the introduction of alkaline 

 waters under great pressure, any mass of old sediments 



