490 PEOF. T. G. BOKKEY AND MAJOR-GEN. C. A. M'^MAHON 



a very high temperature *, that mineral separation had already com- 

 menced, and that it consisted of crystals of felspar and pyroxene f, 

 or — which perhaps is more probable — of completed pyroxenes and 

 inchoate felspars, floating in a magma (having in the latter case the 

 composition of felspar) which was already not very liquid. AVhen 

 the temperature was slightly lowered, as for instance near the faces 

 of a fissure, the magma might become sufficiently viscous to exer- 

 cise considerable strain upon the included crystals ; they would be 

 occasionally cracked, deformed, torn up, and aggregated in streaks ; 

 the mass also would become ill mixed ; in short, it would exhibit on 

 a large scale the phenomena of a fluxion-structure, which would be 

 most conspicuous towards the surface, but might set in here and 

 there in any part, or might occur like a foreign fragment owing to 

 rupture and entanglement of portions of an outer " crust." As 

 consolidation proceeded, the magma would sometimes continue to 

 augment the crystals already formed, and the coarser varieties be 

 produced ; sometimes it would independently crystallize, and thus a 

 fine-grained variety be produced or a quasi-porphyritic condition be 

 retained J. In short, we offer for this rock an explanation which is 

 in some respects similar to that which we have proposed for tlie 

 banded granulites §. At any rate it is an hypothesis which meets 

 all the conditions of the problem at present known to us, and this 

 certainly cannot be said of the dynamo-metamorphic one. 



(3) Varieties of Gahh^o. 



As a rule, except for the above-named mineral changes, the 

 gabbro seems fairly uniform in composition, though varying in 

 structure and in coarseness, but a few exceptional cases have been 

 noted. A dyke, about 2 feet thick, on Enys Head consists mainly 

 of the saussuritic mineral ; one, composed largely of rather well- 

 preserved labradorite, occurs in Lankidden Cove ||, and another was 

 found this year on the north side of Kennack Cove. The last does 



* The coarse condition of the gabbro, even in very thin veins, might be held 

 to indicate a very high temperature and very slow cooling, but this does not, in 

 our opinion, accord so well with some of the other conditions. 



+ Doubtless with olivine and iron oxide, but we omit them as immaterial for 

 the present purpose. 



X Possibly also in some cases, when the gabbro may have been intruded be- 

 tween fault planes, a movement or movements of one or both of the walls of 

 the fault might take place when the final stage of consolidation was setting in, 

 and marginal shearing and foliation would be the result. Some internal 

 shearing might also result from the same cause. 



The coarseness of the rock in the thinner veins may be explained thus : — The 

 front portion of the advancing mass would probably contain rather more 

 crystals (as it would be rather cooler) than the rest. Thus it might for a time 

 be ai-rested at the entrance of a narrow fissure, but when ruptured (owing to 

 the pressure of the mass behind it) the fissure would be filled by a squirt of the 

 more liquid magma, which would sweep along with it the minerals in the more 

 crystallized portiou. 



§ The occurrence of an analogous structure in granite was described, and 

 a similar explanation was suggested, by Gen. M'^Mahou, Geol. Mag.'(1887) p. 76. 



II Quart. Journ. Geol. Soc. vol. xxsiii. (1877) p. 905. 



