390 REPORT—1905. 
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 suf- 
ficiently great to prevent the growing crystals from sinking at once; we have, 
therefore, all the conditions favourable for the growth of porphyritie 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 com- 
pounds and not to alter their order or their nature; he estimates that this displace- 
ment 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 magmas 
pass from the metastable to the labile condition we may use this knowledge to 
determine the exact temperature at which certain of their minerals crystallised. 
Ordinary petrographical descriptions supply numerous examples of the difference 
between the metastable and labile conditions to anyone who will read them in the 
light of the suggestion which | have made; others are to be found in such experi- 
ments as those of Vogt or Doelter. 
My own hope is that when more experiments have been made upon mixed 
supersaturated solutions it will be found that most, if not all, of the features of 
rock development are parallelled by the ordinary processes of crystallisation, but 
that motion, supersaturation, and supercooling are most important factors. 
The very similarity between the differentiation of the alumo-alkaline and 
ferro-magnesian minerals on a small scale in the rock, and that of the alumo- 
alkaline (or salic) and ferro-magnesian (or femic) rocks themselves on a large 
scale, points to some similarity of origin. 
In order to avoid burdening this address with detail I have merely chosen the 
researches of van ’t Hoff, Vogt, Doelter, and Heycock and Neville as illustrations of 
experimental work conducted on the lines of modern physical chemistry, and 
have omitted much that might have been mentioned ; the valuable researches of 
Pelouze, Lagorio, Morozewicz, and Loevinson-Lessing, and the melting-point 
determinations of Joly I have not quoted, because they belong for the most part 
to an earlier period than that which I am considering, and have been discussed 
by Teall and other writers. 
Many very interesting speculations I have passed over entirely, because my 
object has been to focus attention upon experimental evidence. I cannot help 
thinking that these speculations are often based upon chemical actions and 
equilibria that may be impossible; but we cannot criticise them for lack of 
evidence, and I return to my original statement that geology is only beginning to 
enter the experimental stage. 
An earnest beginning is, however, being made. The researches on mineral and 
rock synthesis which I have already quoted are laying a solid foundation ; and I 
see no reason why something of the sort which has been done by van ’t Hoff and his 
collaborators for the aqueous deposits of Stassfurt should not ultimately be worked 
out for an igneous complex, though it may involve tenfold the labour and tenfold 
the time. We have already to welcome the establishment by the United States 
Geological Survey of a laboratory for the express purpose of applying to minerals 
and rocks the exact methods of modern physics and physical chemistry. The 
yery suggestive research of Day and Allen upon the thermal properties of the 
