392 ME. p. ETJTLEY ON ERUPTIVE EOCKS FEOM THE 



33 . On some Eeuptive EocKs/rom the Neighbouehood of St. Minver, 

 CoENWALL. By Feank Rutley, Esq., Lecturer on Mineralogy 

 in the Eoyal School of Mines. (Eead June 9, 1886.) 



[Plate XII.] 



Among the numerous exposures of eruptive rock occurring in the 

 vicinity of St. Minver, near Padstow, those at Cant Hill and 

 Carlion are conspicuously represented on Sheet 30 of the Geological 

 Survey map. 



In both cases these rocks occupy high ground, and in both they 

 are mapped as " greenstone ; " but a casual glance suffices to show 

 that they are very different from one another, although at their 

 nearest points they are little more than half a mile apart. 



The surface of the ground is undulating, and the hills are, as a 

 rule, smooth and under cultivation, so that it seems probable that 

 the boundary lines of these eruptive rocks have been partly mapped 

 from superficial features, the exposures being neither large nor 

 numerous. If it were not for two or three small quarries which 

 have been opened, there would be very little eruptive rock visible, 

 and in no case did I see any contact with the adjacent slates. I am 

 therefore unable to state what is the relation which the former 

 bears to the latter, although, from an inspection of the Survey map, 

 I hoped that it would have been possible to gain some information 

 upon this point at the base of Cant Hill, on the eastern side. 



With this object in view I examined the foot of the hill, passing 

 from St. Michael along the bank of the estuary to the small cove near 

 Cant Farm, but, at the spot where, judging from the map, the 

 eruptive rock might be expected to be visible in place, nothing but 

 loose fragments of it were to be seen. The journey round the base 

 of the hill being dependent on the tide, and the time at my disposal 

 being limited, it seemed scarcely worth while to repeat the visit ; but 

 it is just possible that more diligent search, hereabout, might afford 

 useful information. 



The impression which I first received, on examining sections of 

 the Cant-Hill rocks microscopically, was that they were tuffs 

 composed of basic lavas and fragments of vesicular glassy rock ; but 

 Prof. Eenard, to whom I forwarded the sections, having kindly 

 examined them, favoured me with his opinion that they were vitreous 

 lavas of a diabasic type, and, after much time spent in further 

 examination of them, I have arrived at a similar conclusion. My 

 thanks are therefore largely due to Prof. Eenard for his useful 

 suggestion. I am, however, still not fully satisfied that the clastic 

 appearances visible in some of these rocks may not be due to the 

 presence of volcanic ejectamenta, although it is also possible that 

 these appearances might result from the crushing of a lava by the 

 pressure of superincumbent masses. And here another difficulty 

 steps in, for we sometimes meet with beds of volcanic ejecta 



