170 Proceedings of Boy al Society of Edinburgh, [feb. 20, 
have been greater the nearer we approach to the outside of the 
volcanic mass or to the junction with a large enveloped fragment. 
Even in the contact with such a fragment we have been able to 
show that, proceeding from the enclosed fragment towards the 
central portion of the volcanic body, a constant diminution of the 
twinned augite crystals takes place in favour of the untwinned 
individuals. In explaining the cause of these interesting facts, there 
is ample ground for assuming that all these analogous phenomena 
are to be considered as consequent upon sudden cooling. Just as 
glass rapidly cooled down proves to be brittle, and exhibits anom- 
alous optic properties, so the volcanic rock, rapidly cooled down 
near the junction with cold associated rocks, will likewise be subject 
to a molecular tension. This tension, I believe, manifests itself 
by producing twinning both in augite and felspar, as well as by 
conferring anomalous properties on the quartz. I am unable to 
offer a suitable physical explanation of these phenomena, and can only 
offer the following suggestion : — The porphyritic crystals scattered 
through the microporphyritic diabase must have separated a long 
time before the consolidation of the ground-mass. Therefore the 
strain manifested in the twinning of the porphyritic crystals must 
have had its origin in those portions of the magma which, after the 
consolidation of the whole mass, represent the ground-mass. The 
latter had alone been directly influenced by the rapid cooling. 
There first resulted a contraction from the cooling of the ground- 
mass, and then this contraction extending to the porphyritic crystals, 
seized on the crystal at some point of its outer planes, with the 
tendency to draw it asunder. 
To sum up these results, we may say that the conditions, which we 
must postulate for the strain, correspond with the effects we have 
observed in different minerals. The conclusions as to the effect of 
this tractive power, that we were able to draw from either mineral, 
represent, therefore, three views. These fairly coinciding one with 
another, point to one hypothesis, which thus assumes a high degree 
of probability. 
The results we have arrived at agree wuth the facts that 
Prof. Judd* has noticed, in his paper “On the Tertiary and 
older Peridotites of Scotland.” There, however. Prof. Judd drew 
* Qua7't. Jour. Geol. Soc., 1885, xli. pp. 354-418. 
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