528 Correspondence — F. P. Mennell and C. G. S. Sandherg. 



C0I^iaE:S]P03SriDEIT0E. 



ORIGIN OF THE IGNEOUS EOCKS. 

 Sib,, — I was much interested to read in your last issue the excellent 

 review of Professor's Daly's latest work appearing above the initials 

 " R. H. E,." May I draw your reviewer's attention to the fact that 

 such points as the quantitative composition of the igneous rocks and 

 the effects of assimilation upon them have been dealt with at some 

 length in England before now, as your own pages can testify. In this 

 Magazine as long ago as June, 1904, I entered a protest against the 

 calculations made on a purely arm-chair basis by F. W. Clarke and 

 A. Harker with regard to the former problem, and Mr. Clarke has, 

 to some extent, modified his calculations as a result of this and 

 a succeeding paper on the same subject. With regard to assimilation, 

 a process intimately connected with the question of composition, 

 Professor Grenville Cole has been crying in the wilderness for many 

 years past, and the late Dr. Jolmston-Lavis has done good work in the 

 same direction, while I attacked the problem on a much wider basis 

 in a lengthy paper read before the British Association in 1905. This 

 has been followed up by a general work on petrology, of which an 

 expanded form was published by Messrs. Chapman & Hall last year. 

 In substance it is a systematic exposition of the subject based on 

 assimilation as opposed to the purely hypothetical ' diflFerentiation ' 

 basis which has become so popular of late years. It may, perhaps, 

 be added that Professor Daly has placed a great stumbling-block 

 before many geologists by his advocacy of a prevailing basic magma, 

 below the granitic one, as a primary feature of his work. 

 49 London Wall, E.G., Oct. 6,1914. F. P. Mennell. 



THE OLD OR GREY GRANITE OP THE TRANSVAAL AND 

 ORANGE RIVER COLONY. 



Sir, — On May 13 of this vear a paper was read in the Geological 

 Society of London by P. W. Penny, B.Sc, P.G.S., "On the 

 Kelationship of the Vredefort Granite to the Witwatersrand System" 

 (see Geol. Mag., July, 1914, pp. 332-3), wherein the writer suggests 

 •'that the Vredefort Granite, instead of being 'Archaean', is of a post- 

 Pretoria - pre-Karroo age, if not contemporaneous with, at least 

 connected with, the same epoch of igneous activity as the ' Red 

 Granite' of the Northern Transvaal". 



Thus nearly six years after the publication of my paper on " The 

 Age of the ' Old or Grey Granite ' of the Transvaal and Orange River 

 Colony" in the Geol. Mag., Dec. Y, Vol. V, JSTo. 534, pp. 552-9, 

 another serious investigator arrives at the identical conclusion, 

 evidently quite independently, as Mr. Penny, although quoting 

 various authors, seems to have been quite unaware of the existence 

 of ray paper above mentioned. 



I still hold the firm conviction that the adoption of the above 

 detailed conclusion, although revolutionizing to a large extent all that 

 has hitherto been accepted, will prove the only way to place South 

 African geology on a sound and true foundation. 

 Haaklem, Holland, Sept. 21, 1914. C. G. S. Sandbekg. 



