Correspondence — C. E. Tilley. 235 



Sir, — Mr. Dixey's sjiecial pleading for the use of the term 

 ^'magnesian", and for the setting up of a third great group of igneous 

 rocks co-ordinate with the alkaline and calc-alkaline suites, carries 

 little conviction in its train. 



The impropriety of the term " raagnesian " is sufficiently evident 

 on inspection of a series of first-class analyses of rocks of the 

 charnockite series, from any of the well-defined provinces such as 

 those of Southern India or Western Norway. 



The ground of Mr. Dixey's paper was largely covered by H. S. 

 Washington in 1916 in his paper on " The Charnockite Series of 

 Igneous Rocks " {Amer, Journ. Sci., vol. xli, pp. 323-38), but with 

 somewhat different conclusions. In summing up the chemical 

 characters of charnockite provinces, he was led to state that '" they 

 are characterized by the dominance of iron oxides over magnesia 

 and lime, the two latter being present in about equal amount ". 



One word with regard to the concluding paragraph of Mr. Dixey's 

 letter. Mr. Dixey has missed the point of my remarks if he has not 

 perceived that the reaction discussed was in no way dependent on 

 any intrusion of norite by a later member of the series. 



The cordierite-norites of Minnesota described by Winchell {Amer. 

 Geol., vol. xxvi, 1900, p. 151) afford an even more instructive 

 illustration of the development of hypersthene than those of the 

 Huntly area cited. The normal gabbro from which the cordierite- 

 Eorites are developed by assimilation of alumiuous sedimentary 

 material, is free from rhombic 2:)yroxene, while the cordierite- 

 norites ore free from the monoclinic type. 



The inaccuracy of the reaction I have stated for the disappearance 

 of diopside, when oordierite appears, can be admitted when Mr. Dixey 

 can produce cordierite-norites, which, apart from armoured relics, 

 contain monoclinic pyroxene, as in the gabbros or norites with 

 which they are associated. 



C. E. Tilley. 



THE ENGLISH ESKERS. 



Sir, — Professor Gregory scores. The passage quoted by me from 

 the Geological Sarvey Memoir on the Yorkshire Coalileld does 

 refer, I agree, to mounds which only in part belong to the Lanshaw 

 Delves series, and not at all to the Delves themselves. Toicche ! 

 The fact remains, however, that Carvill Lewis speci.Qcally 

 mentions " Laneshaw Delves, and limekilns have been built upon 

 them '". So Professor Gregory is honourably acquitted of ignoring 

 two statements of the fact ; he ignored only one. I will not labour 

 the point of whether Passell's " four-tenths " of the great Memoir 

 on the Yorkshire Coalfield includes the passage under discussion, 

 but content myself with remarking that the quaint " harbour-bar" 

 hypothesis for the Bingley Mounds is found also in his early paper 

 in the J^.A.. report for 1873. 



