422 Correspondence — H. A. Baker. 



type connected with the Devonian movements, and gives no 

 indication of entering the Carboniferous. Microscopic examination 

 of contact-altered rock suggests that the cleavage is later than the 

 recrystallization. Thus the granite is older than the chief move- 

 ments (Devonian). It is associated with an anticline demonstrably 

 pre-Bala, running oblique to the Devonian folding, and is therefore 

 probably itself pre-rBala and of the same age as the surrounding 

 intrusions which belong to the Borrowdale suite. 



The following lecture was delivered: — 



" Diagrams illustrating the Significance of Rock Analyses." By 

 John William Evans, D.Sc, LL.B., F.G.S. 



The diagrams ai-e of two kinds: (1) Individual rock diagrams 

 intended to indicate at a glance the significance of the analyses of 

 a rock or complex mineral silicate ; (2) Linear rock diagrams. The 

 different types of linear or variation diagrams, in which the chemical 

 constituents of different rocks are represented by vertical distances, 

 were reviewed and a new type proposed in which each rock is 

 represented by two diagrams : («) alumina diagram, {b) silica 

 diagram. 



In describing these diagrams, the lecturer discussed various 

 problems connected with the genesis and composition of igneous 

 rocks. 



2. July 5, 1918.— J. F. ^\ Green, B.A., F.G.S., President, in the 



Chair. 



The following lecture was delivered : — 



" A Yisit to Christmas Island and the Cocos-Keeling Islands." 

 By C. W. Andrews, D.Sc, F.R.S., F.G.S. 



The structure and physical geography of Christmas Island (Indian 

 Ocean), a raised coral island, was described and compared with 

 those of the Cocos-Keeling Islands, a typical atoll. Some account 

 of the fauna and fl.ora of Christmas Island was given, with special 

 reference to the means of colonization of oceanic islands. 



The lecture was illustrated by lantern slides. 



coiRK-Esi^onsriDEJisrcE]. 



THE PEE-THANETIAN EEOSION OF THE CHALK. 

 SiE, — I should like to express to Mr. C. N. Bromehead my 

 grateful thanks for his criticism of my paper on the Pre-Thanetian 

 Erosion of the Chalk of tlie London Basin. I am afraid I must 

 plead guilty to the charge of not having made use of all the evidence 

 available, in ignorance as I was of the appearance of the Geological 

 Survey Memoir on The Geologxj of Windsor and Chertsey. This 

 memoir has, I suppose, been published since the outbreak of war 

 and I have been continuously on active service, first flying in 

 France and later serving afloat, since 1915. In any geological work 

 with which I endeavour to beguile the tedium of life afloat I am 

 greatly handicapped by being unable to make references to authorities, 

 or even to consult earlier work of my own. The paper under present 

 discussion was simply the outcome of an attempt on my part to apply 

 cartographic methods to the data in my possession bearing upon the 



