334 Reports and Proceedings — Geological Society of London. 



older porphyrites (altered andesites) which cover so large an area 

 in the district. The breccias contain enormous numbers of blocks 

 of a red dacite (quartz-andesite), and inclosed in this rock angular 

 fragments of a glassy rock, resembling a " pitchstone-porphyry," are 

 found, everywhere, however, more or less converted into a white 

 decomposition-product. The youngest igneous rocks of the district 

 are the bosses and dykes of melaphyre (altered basalt and dolerite), 

 which have been often so far removed by weathering as to leave 

 open fissures. 



In the Appendix three very interesting rocks were described in 

 detail. The rock of the Northfield Quarry, which is shown to be 

 an augite-andesite, has a large quantity of a glassy base with felted 

 microlites, and contains large porphyritic crystals of a colourless 

 augite. The rock of the Causewayhead Quarries is described as an 

 enstatite-andesite ; it has but little glassy base, being made up of 

 lath-shaped felspar crystals (andesine), with prismatic crystals and 

 grains of a slightly ferriferous enstatite ; there are no porphyritic 

 crystals, but the enstatite individuals are sometimes curiously aggre- 

 gated. The red porphyritic rock from the breccias near the Tay 

 Bridge was shown to be a mica-dacite, and the glassy rock asso- 

 ciated with it to be the same material with a vitreous in place of a 

 stony base. This glassy base exhibits very beautiful fluidal and 

 perlitic structures. The crystals of first consolidation in this rock 

 are oligoclase and biotite, often showing marks of injury in trans- 

 port ; those of the second consolidation appear to be orthoclase. In 

 conclusion, the successive stages by which the andesitic rocks of the 

 area were altered, so as to assume the characters distinctive of por- 

 phyrites, were fully discussed, as well as the change of the glassy 

 rock into its white decomposition-product. 



2. " On some Eruptive Rocks from the Neighbourhood of St. 

 Minver, Cornwall." By Frank Rutley, Esq., E.G.S. 



The rocks described in this paper were derived from Cant Hill, 

 opposite Padstow, and from a small quarry about half a mile from 

 Cant Hill, near Carlion. At the former locality the volcanic rocks 

 are much decomposed, but from their microscopic characters they 

 may be regarded as altered glassy lavas of a more or less basic type. 

 No unaltered pyroxene, amphibole, or olivine is to be detected in the 

 specimens described, but there is a considerable amount of secondary 

 matter which may include kaolin, serpentine, chlorite, palagonitic 

 substances, etc. There is evidence of fluxion-structure in some of 

 the sections ; others are vesicular, and the vesicles are usually filled 

 with siliceous or serpentinous matter. The relation of these lavas 

 to the underlying Devonian slates was not ascertained. The rock 

 occurring near Carlion contains numerous porphyritic crystals of 

 augite, in which the crystallization is interrupted by the co-develop- 

 ment of small felspar crystals, which appear, as a rule, to have been 

 converted into felsitic matter. Ilmenite is also present in patches 

 which indicate a similar interrupted crystallization to that shown by 

 the augite. The rock has the mineral constitution of an augite- 

 andesite ; but since it is a holocrystalline rock, exception would be 



