20 . Bev. R. Baron — Rock Cavities in Granite. 



imprisoned vapours in the original rock magma. We know what an 

 important role the occlusion of gas and liquid bubbles plays in 

 individual minerals ; to how large an extent bubbles, many of them 

 gigantic, of steam and other gases rise from the depths to the surface 

 during volcanic activity ; and how also solidified lava is frequently 

 riddled with rounded vesicles, the former receptacles of imprisoned 

 gas. Is it therefore improbable that the hot vapours in certain 

 deep-seated magmas, instead of being entirely absorbed, have, in 

 certain parts and under certain conditions, so pushed aside the 

 viscous mass by their expansive force as to form larger or smaller 

 vesicles ? And, further, is there any reason against the supposition 

 that some of these spaces have been occupied by several bubbles, 

 for a time at least, in juxtaposition, though separated probably by 

 partitions of the magmatic material ? The existence of protruding 

 ridges (which are sometimes present) would seem to support such 

 a view, the remaining walls having broken down and disappeared. 

 If this hypothesis be correct, it would seem to follow that these 

 gas cavities, or some of them, must have existed during at least 

 the closing stages of the solidification of the rock ; in other words, 

 from the time that the rock was still sufficiently viscous to permit 

 of being pushed aside by the expansive power of the vapour, until 

 it had hardened to a degree sufficient to prevent the collapse of the 

 walls of the cavities. Whether previously the whole of the gas 

 was equally dispersed throughout the rock mass, or whether part 

 of it had accumulated here and there into bubbles, I do not pretend 

 to say. The diminution of pressure during the final stages of 

 solidification would probably also allow of the increased expansion 

 of the imprisoned gases. It would also follow that, as denudation 

 proceeded at the surface of these rock masses, fresh cavities should 

 continually come to the surface, and that this is the case seems to 

 be shown by the smallness of the entrance to some of the holes 

 (Professor Bonney speaks of one only two inches in diameter),^ 

 entrances which would naturally widen until the hole would become 

 more or less basin-shaped, and finally disappear. 



What are the miarolitic spaces and drusy cavities which not 

 unfrequently are to be found in plutonic rock masses ? Are these 

 in every case due to contraction on crystallization of the rock 

 material, or may they not be sometimes owing to the expansion 

 of localized vapour ? If the latter, these rock cavities may be 

 nothing more than miarolitic spaces or druses. 



It is in consequence of the improbability of the production of 

 these rock cavities by any form of superficial action suggested or 

 apparently conceivable that I have been led to the above hypothesis 

 as their efficient cause. What are the objections to it ? 



^ Mr. Barou has forgotten that this little hole was merely one of communication 

 between two adjacent canties, the inner of which had one of ordinary size lower 

 down. — Ed. Geol. Mag. 



