240 Correspondence — Major- General McMalton — Mr. Harker. 



ground of the much gi'eater probability of a directly mineral oriijin 

 of such limestones, as the necessary result of chemical reactions, 

 which a common-sense application of known laws of thermal and 

 general chemistry tells us must have taken place in the earlier 

 (" pre-oceanic") stage of the history of our earth. This was put 

 plainly enough before the geological world in my " Metamorphism 

 of Rocks" (Longmans, 1889), pjiges 6-16; and it is needless that 

 I should do more now than refer the reader to that work, so far as 

 concerns the theoretical bearings of the facts here nari-ated. 



Malvern. A. IrvING. 



Uth April, 1892. 



REPLY TO PROF. J. F. BLAKE. 



Sir, — There is only one point in Prof. Blake's reply in your 

 April Number that I intend to notice. Prof. Blake writes : — 

 "General McMahon says he was considering capillary flow under 

 heat and pressure, but in his paper he really only discusses the action 

 of heat, and the present discussion on the effect of pressure is a 

 new one." This statement is really a very extraordinary one. In 

 my paper in the Geological Magazine (February, 1892, pp. 74, 75). 

 I simply refer to statements regarding pressure made in my original 

 paper (Proc. Geol. Assoc, vol. xi. pp. 431, 432). In this last paper 

 I showed that pressure was a very important factor, and I gave very 

 interesting statistics at pp. 438, 439 (then published for the first 

 time) supplied to me by an eminent engineer showing that in the 

 case stated, the actual measured pressui-e at 190 feet below the 

 surface, was equal to the calculated pressure, and was no less than 

 80 lbs. on the square inch. 



I do hope for the future success of " The Annals of British 

 Geology," that Prof. Blake will devote a little more attention to the 

 mastery of the papers he attempts to boil down. Unless he does so, 

 I am afraid that his geological Bovril will not prove a very stimu- 

 lating or nourishing article. 



20, Nevern Square, C. A, McMahon, Major-General. 



IQth April, 1892. 



CONE-IN-CONE STRUCTURE. 



Sir, — If Mr. Young's statement* is meant to be of universal ai^plication, 

 it is certainly not borne out by observation. A radial arrangement of the 

 cones about a large nodule is, I believe, not an uncommon thing. Good 

 examples occur in the Lingula Flags of Berth near Portmadoc, which 

 contain flattened nodules, extending along the bedding, sometimes several 

 feet long. Each is surrounded by a layer of well-characterized "cone- in- 

 cone," and the apices of the cones are directed inwai'ds towards the nodule, 

 so that they point downward on the upper side, upward on the lower side, 

 and horizontally on the edges of the nodule. 1 have noticed the same 

 thing on a smaller scale in the shales of the Yorkshire Lias. 



St. John's Coll. Camb. Alfred Harker. 



*^* The Editor regrets that, through inadvertence, this letter has been delayed 

 in publication. — Edit. Geol. Mag. 



1 See Geol. Mag. for March last, p. 138. 



