104 PROF. T. G. BONNEr ON A CONTACT-STETTCTUEE 



perature were rapidly lowered and the process of crystal-building 

 stopped, the result would be a porphyritic rock ; while if it were 

 slowly cooled this structure might not be developed. It is evident 

 sometimes that the porphyritic structure has been set up during 

 the final consolidation of the rock, as for example in a dyke, where 

 the crystals are wanting near its exterior, set in gradually, and get 

 larger towards the middle; sometimes it is no less evident that 

 these crystals belong to an earlier stage in the history of the rock, 

 and that, whether lava-stream or intrusive mass, it consisted, on 

 assuming its present position, of solids (^. e. crystals) embedded in 

 a more or less viscid fluid. The latter may have been in any con- 

 dition, from a nearly perfect liquidity to one like that of putty or 

 cooling tar. Eut at this stage the crystals previously formed may 

 be exposed to strains, and so are liable to be fractured. Of course, the 

 temperature of the mass, ceteris paribus, will be then comparatively 

 low. 



jSTow it is obvious that the readiness with which a glass is formed 

 depends not only on the circumstances of cooling, but also on the 

 composition of the magma ; e. g, large masses of glass are very rare 

 among the more basic rocks, and perhaps unknown among peridotites. 

 Moreover, among the acid rocks, we find that in some the glass is 

 comparatively free from microliths, while in others it is crowded 

 with them (felspar usually), sometimes to such an extent that the 

 presence of a vitreous base can only be demonstrated in very thin 

 sections. Suppose, then, a mass be cooling, which is composed of 

 crystals, some of which are felspar, scattered throughout a magma 

 which consists of the constituents required in forming a felspar, 

 together with an excess of silica. This magma in certain cases 

 may solidify as a glass; in others the felspar may be gradually 

 separated, until ultimately the residue is silica, which will then 

 crystallize as quartz *, Now this diff'erentiation of the magma may 

 take place in any of three ways : — (1) The felspathic constituents 

 may be added to the felspar already existing, and the residue 

 crystallize as interstitial quartz, which may be sometimes of con- 

 siderable size f. This process supposes considerable freedom of 

 molecular movement : that is, probably, a very slow change. (2) In 

 some cases the magma (probably if it is abundant) may setup an inde- 

 pendent holocrystal]ine structure (as is the case in most porphyritic 

 granites), in which, as a rule, the smaller felspars are also idiomorphic. 

 (15) In other cases the felspar, though it separate from the quartz and 

 crystallize, may be forced, as it were, to include the quartz ; still 

 both the one and the other, though each forms a kind of lattice-work 



* Of course I do not forget that water is present, but as this has no direct 

 bearing on my line of thought (though it is a most important factor) I do not 

 mention it. 



t In some cases we find little grains or even vein-like masses of a felspathic min- 

 eral which appears to have been occluded from the quartz. In this case probably 

 the felspathic constituents were not perfectly segregated from the residue when 

 free molecular movement became impossible. Eut sometimes the felspar forms 

 with the quartz a structure which might be called ' ophitic,' the quartz being 

 analogous in its mode ol" occurrence to the augite v.\ this variety of dolerite. 



