34 GEOLOGY. 



Phillips, J. A. The Bocks of the Mining District of Cornwall, and 

 their Ilelation to Metalliferous Deposits. Quart. Journ. Geol. Soc. 

 vol. xxxi. pp. 319-345, \)\. xvi. (microscopic structure of rocks). 

 The Killas is of Devonian or Carboniferous age, with the exception 

 of some in the S.W., which is probably Silurian. The Granite is post- 

 Carboniferous. Interbedded with the Killas at Trelill, St. Kew, there is 

 a mass of altered doleritic lava. Trap- dykes traverse the Killas in 

 different parts ; but many of the rocks which have been mapped as 

 ' greenstones ' are shown to be altered hornblendic slates. The Granite 

 is usually coarse-grained ; but near the Killas it becomes finer, and it 

 frequently sends off veins into the adjoining slates. Coarse-grained 

 granites are sometimes traversed by granitic veins of a finer texture. 

 The felspar is mostly orthoclase ; but a plagioclastic species, supposed 

 to be albite, is frequently present. The mica is sometimes replaced by 

 a talc-like mineral. Schorl is almost invariably present. The dykes 

 of Elvan traverse slate and granite alike ; but th,ey are most numerous in 

 the latter. The elvans have the same ultimate composition as the 

 granite ; but the aggregation of their constituents is different. The 

 author doubts whether the fluid cavities of the crystals in the granites 

 and elvans yield information that can be relied upon as to the tempera- 

 ture at which the crystals w^ere formed. He shows, from a number of 

 analyses, that the granite cannot be regarded as extremely metamor- 

 phosed killas. The paper concludes with a discussion of the origin of 

 mineral lodes. The granite being post-Carboniferous, and the elvans 

 subsequent to the granite, it is shown that the vein-fissures are later 

 than either. These fissures were subsequently filled by chemical action, 

 from heated water and aqueous vapours circulating through them, 

 lateral infiltration greatly infiuencing the results. Contact-deposits, 

 stockwerks, and the alteration of stratified deposits near eruptive rocks 

 are probably due to analogous chemical action. ^ W. T. 



Prestwich, Prof. J. Notes on the Phenomena of the Quaternary 

 Period in the Isle of Portland and around Weymouth. Quart, 

 Journ. Geol. Soc. vol. xxxi. pp. 29-54, plate i. (map and sections). 

 8 woodcuts (sections and fossils). 

 The oldest Drift of the district is that represented by the patch of 

 mammaliferous gravel near the Portland Prison; this contains Elephas 

 antiquus, and is partly composed of pebbles derived from the Green- 

 sands and Tertiaries on the I^. The author believes that this gravel 

 was deposited when there was a continuous slope of land from the 

 Eidgeway to Portland, and before the anticlinal which has elevated the 

 strata of the intermediate area was formed ; the Eidgeway fault is of 

 older date. The raised beach of Portland Bill is mainly composed of 

 flints, but it contains other pebbles of local origin as well as some which 

 have travelled from the "W. Overlying the beach there are deposits of 

 sand, loam, and angular rock-debris. The beach is of old date ; but the 

 overlying beds are subsequent to the anticlinal. The author thinks 

 ,that these later beds were formed during a depression of the land, when 



