460 Notices of Memoirs — 



and it is material to note that it was the first mineral to crystallise 

 out of the magma of the Satlej granite. This is shown by several 

 circumstances. 



In the first place, the beryl preserved its perfect crystallographic 

 shape, showing that its molecules during the entire period of 

 crystallisation possessed comparative freedom of motion, and were 

 not interfered with or molested by other solid minerals. In the 

 second place, all the essential minerals of the granite when they 

 subsequently crystallised out of the magma were deposited on the 

 crystals of beryl. I have specimens of the granite showing crystals 

 of beryl enclosed in felspar, in muscovite, and in quartz. 



The beryl, therefore, having been the first mineral to crystallise, 

 the examination of thin slices of it under the microscope ought to 

 give us a clue to the condition of the magma at the time the beryl 

 was formed. I have made such an examination, and I find that the 

 beryl is crowded with liquid and gas cavities, the former containing 

 movable bubbles and deposited crystals as well as water. The 

 bubbles are of substantial size relative to the area of the cavities, 

 showing that the water sufi"ered considerable contraction after it was 

 sealed up in the beryl. 



Scrope long ago suggested that the fluidity of lavas below the 

 melting-point was due chiefly to the water they contained, and 

 attributed the liquidity of granite to the same cause. Scrope, how- 

 ever, in ascribing the mobility of an igneous rock to the presence of 

 water, seems to have had regard principally or wholly to its 

 mechanical action in furnishing an elastic medium in the interstices 

 between the crystals or grains of the rock. He observes that a lava 

 consists " of more or less granular or crystalline matter, containing 

 minute quantities of either red-hot water, or steam in a state of 

 extreme condensation, and consequent tension, disseminated inter- 

 stitially among the crystals or granules, so as to communicate 

 a certain mobility to them, and an imperfect liquidity to the com- 

 pound itself," and he quotes Scheerer and Delesse, both of whom 

 assert that water exists in mechanical combination with all crystalline 

 rocks, "its minute molecules being intercalated between the 

 crystals." 



Nowadays one would attribute the liquidity of an igneous rock 

 not so much to the mechanical action of the water present in it as 

 to the combination of the water with the mineral contents of the 

 lava, producing a state of solution. Sorby's investigations supported 

 Scrope's observations, for he proved that the liquid contained in 

 the inclusions in granite is water, and showed that it was caught 

 up during the formation of the crystals, "and was not introduced 

 subsequent to the consolidation of the rock." The water now con- 

 tained in cavities in the beryl was probably held in solution by the 

 constituents of that mineral at the time of its formation, and as it 

 cooled down the water separated from the substance of the beryl 

 and formed the cavities in which we now find it imprisoned. If 

 this be so, it follows that when the beryl crystallised out of the 

 magma, the latter was in a fluid condition, and held a considerable 



