124 H. S. JEVONS, H. I. JENSEN AND C, A. SUSSMILCH. 



never, of course, an open crack, the mother liquor exuded 

 always filling the plane of parting ; because the contraction 

 due to the cooling of the mass as a whole, which results 

 both from crystallisation and from cooling, is probably taken 

 up during the earlier stages of cooling, by the pressure of 

 the superincumbent strata. During the later stages of 

 consolidation and during subsequent cooling further con- 

 traction leaves actual spaces between the crystals, — the 

 so-called miarolitic cavities. This name has been applied, 

 not only to intercrystal spaces, but also to the larger 

 irregular cavities sometimes occurring in plutonic rocks, 

 which I regard as pockets of aqueous and other vapours, or 

 of watery solution miscible with the magma. They are, I 

 believe, simply plutonic steamholes, of the same nature as 

 the original cavities of amygdaloidal lavas; the conditions 

 giving rise to them are rarer. 



The common texture of aplitic rocks is well accounted 

 for by this theory of their origin. There must be a slow 

 but constant motion in the liquid whilst it is slowly aggre- 

 gating in the cracks from the surrounding mass, and this 

 liquid is all the time slowly crystallising. A constant slow 

 movement, continuing almost to the completion of crystal- 

 lisation, is the best explanation of the origin of an even- 

 grained allotriomorphic texture. Crystallisation would 

 probably at any one moment be slightly more advanced in 

 the centre part of the vein than at the sides which were 

 being recruited with fresh mother liquor, but such a time- 

 difference need not have any appreciable effect upon the 

 texture of the solid rock. The theory which I have just 

 stated is the one which has for many years seemed to me 

 to give the best explanation of the phenomena of aplitic 

 veins, and I have had it in mind when examining numerous 

 examples of aplitic veins in the field, and I have seen no 

 occurrences which were in any way inconsistent with it. 



