78 Geological Society, 



rock associated 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 transport ; 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 cha- 

 racters distinctive of porphyrites, 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, &c. 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 

 taken by many petrologists to the employment of the term andesite. 

 The lavas of Cant Hill were also probably of an andesitic character, 

 so that, so far as original mineral constitution is concerned, there is 

 some apparent justification for the mapping of both of these rocks 

 as " greenstone " by the Geological Survey. 



3. " The Bagshot Beds of the London Basin." By H. W. Monck- 

 ton, Esq.,E.G.S., and R. S. Herries, Esq., B.A., E.G.S. 



The authors stated that their object was to describe more fully the 

 Lower Bagshot beds, and to disprove the view lately advanced by 

 Mr. Irving that, in certain places, the Upper Bagshots overlap the 

 Lower and rest directly on " the London Clay. They described or 

 referred to a number of sections all round the main mass, beginning 

 at St. Ann's Hill, Chertsey, where they considered that the mass of 

 pebbles and associated greensands must be referred to the Middle 

 Bagshot. The outliers near Bracknell and Wokingham were shown 

 to consist of Lower and not Middle Bagshot, which does not appear 

 in the valley north of Wellington College. 



The Aldershot district was explained, and it was shown that the 

 beds there resting on the London Clay were Lower and not Middle 



