Reports & Proceedings — Geological Society of London. 285 



diverted to the Black water and the Crouch left as a beheaded estuary 

 or fohrde. 



(6) The Lower Thames and Essex river sj'stems are therefore due 

 to the Eocene earth-movements which formed the London Basin, and 

 tlie coeval uplift of the En<z;lish Midlands started thence a radial 

 drainage. The streams to the south-east cut the wind-gaps on the 

 Chiltern Hills, and the drainage to the south-west flowed along 

 a subsidence on the north-western side of the Jurassic escarpment as 

 the Warwickshire Avon and the Lower Severn. 



2. " The Topaz-bearing Hocks of Gunong Bakau (Federated Malay 

 States)." By John Brooke Scrivenor, ALA., F.G.S. 



Gunong Bakau is a peak, 4,426 feet high, in the main range of the 

 Alalay Peninsula. It is composed of porphyritic granite, into which 

 have been intruded veins of quartz-topaz rock, and, at a later date, 

 masses and veins of topaz-aplite. 



The quartz-topaz rock has quartz and topaz as constant constituents. 

 Other important constituents, which, however, are not always found, 

 are cassiterite, zinnwaldite rich in iron and showing the axial figure 

 of a uniaxial mineral, and tourmaline. The zinnwaldite is only 

 known to occur in quantity in one vein : elsewhere it is sometimes 

 found forming patches in the quartz-topaz rock. 



The topaz-aplite contains a small amount of cassiterite. 



Where the quartz-topaz veins cut the granite a ' reaction border ' 

 of schorl-rock, and, in one case, of greisen, is found. These reaction 

 borders differ wideh' from the veins themselves. 



Evidence is given in detail, showing that the quartz-topaz vein-rock 

 is not an alteration product of a pre-existing rock, but was intruded 

 as a quartz-topaz magma. 



Ore-bodies formed by pneumatolytic alteration of granitic rocks 

 were once worked on Gunong Bakau, and they differed markedly 

 from the quartz-topaz rock. 



It is believed that the difference between the familiar pneumatolytic 

 products, schorl-rock, and greisen, on tlie one hand, and the quartz- 

 topaz rock on the other, is that, in the former case, rocks that had 

 consolidated on the edge of a granite-mass were altered by media 

 coming from deeper parts of the mass ; whereas, in tlie latter, an 

 accumulation of similar media attacked part of the still molten 

 magma deep down in the igneous mass, and the heat generated by 

 the reactions that took place caiiseil the portion of the magma attacked 

 to boil up and consolidate in the granite as an intrusive vein-rock. 

 Segregation of the first-formed minerals during cooling led to the 

 irregular distribution of the constituent minerals in the veins. 



2. April 29, 1914.— Dr. A. Smith Woodward, F.K.S., Bresident, 

 and afterwards, William Hill, Vice-President, in the Chair. 



The following communications were read : — 



1. "On the Lower Jaw of an Anthropoid Ape {Dr)/opithecHs) from 

 tlie Upper Miocene of Lerida (Spain)." By Arthur Smith Woodward, 

 LL.D., F.K.S., Pres.G.S. 



tl 



