522 Reports and Proceedings — British Aasociatioit. 



These events take place in all the aqueous solutions which I have 

 examined, and I am surprised that they have not been discovered 

 before. They afford a possible explanation of two common features 

 of igneous rocks, and of slags — namely, the growth of comparatively 

 lai^ge and isolated porphyritic crystals, or phenocrysts, and the 

 appearance of the same mineral at two or more different periods. 

 The origin and the arrested growth of phenocrysts have generally 

 been attributed to sudden change of temperature, of pressure, or of 

 hydration, and no other plausible explanation has been given, 

 although, as has been sometimes pointed out, they may occur in 

 batholites where there is no independent evidence of such changes. 

 Pirsson has recognised the utter impossibility of the ordinary theory, 

 and has recently suggested that each mineral has its crystallisation 

 interval during which it continues to grow, and that this is 

 terminated by the increasing viscosity of the magma, which checks 

 the supply of further material to the growing phenocrysts and 

 establishes new centres of crystallisation. A similar explanation 

 was adopted by Crosby for the quartz-porphyry of the Blue Hills. 

 He expresses it by saying that owing to the increased viscosity the 

 rate of cooling overtaxes the molecular flow, which cannot keep pace 

 with the crystallisation. It is so difficult to find any satisfactory 

 theory for the growth of phenocrysts that they have even been 

 attributed to the effect of earthquake shocks. 



Now in a silicate magma, in all probability, the temperature is 

 sufficiently high to be that of the metastable condition, the rate of 

 cooling sufficiently slow to keep the liquid in that condition for 

 a considerable time, and the viscosity sufficiently great to prevent 

 the growing crystals from sinking at once ; we have, therefore, all 

 the conditions favourable for the growth of forpliyvitic crystals; 

 these must have generally originated throughout the liquid as 

 spontaneous nuclei if the magma entered the labile state, or may 

 have been started by inoculation or cooling at the margin if the 

 magma as a whole remained in the metastable state. In the latter 

 case suppose that further somewhat sudden cooling brings the 

 magma to the labile condition, then there will be a sudden and 

 spontaneous second growth of nuclei which will not be able to 

 attain the dimensions of the porphyritic crystals; we have here all 

 the conditions necessary for a second generation of one of the 

 constituents of the rock. 



It is not necessary, therefore, to suppose that changes of pressure 

 played any very great part in these matters. I believe it will be 

 found that considerations of temperature and solubility are far more 

 important. Similarly, in the case of the salt deposits van 't Hoff 

 came to the conclusion that practically the only effect of changes of 

 pressure is to displace the temperature of formation of the various 

 compounds and not to alter their order or their nature; he estimates 

 that this displacement is comparable with that of the melting-points 

 under the same agency, and in the case of the calcium-magnesium 

 chlorides only amounts to a few thousandths of a degree for one 

 atmosphere of pressure. 



Perhaps when we can ascertain the temperature at which silicate 



