160 Geological Society. 



(5) A post- Glacial change led to the deepening of the Chelmer 

 Valley between Chelmsford and Maldon, whereby the Chelmer was 

 diverted to the Blackwater and the Crouch left as a beheaded 

 estuary or f 6 h r d e . 



(6) The Lower Thames and Essex river-systems are, therefore, 

 due to the Eocene earth-movements which formed the London 

 Basin ; and the coeval uplift of the English 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 Rocks of G-unong Bakau (Federated 

 Malay States).' By John Brooke Scrivenor, M.A., F.G.S. 



Gunong Bakau is a peak, 4426 feet high, in the Main Range of 

 the Malay 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 con- 

 stituents. 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 : else- 

 where 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 widely 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 pneumato- 

 lytic products, schorl-rock, and greisen, on the 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 the 

 latter, an accumulation of similar media attacked part of the still 

 molten magma deep down in the igneous mass, and the heat gene- 

 rated by the reactions that took place caused 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. 



