492 General McMahon — Further Remarks on. Granite. 



the proper direction, and this may account for its great development 

 in Trilobites. The small size of the masticatory appendages, as in 

 Phyllopods, would also support this view.^ 



The pygidium may have been of considerable assistance as a tail 

 fin, and its increased size in post-Cambrian times would thus be 

 accounted for. 



This comparison does not claim, however, that the Trilobites 

 have near affinities with the Phyllopoda, only that Trilobites had 

 a Phyllopodan mode of life. The Trilobites are the most primitive 

 Crustaceans known, the Phyllopoda are the most primitive Entomo- 

 straca. Nearer than this we cannot at present go. 



IV. — Some Further Ebmarks on Granite : a Keply. 

 By Lieut. -General C. A. McMahon, F.R.S., F.G.S. 



AFTER the expiration of twelve months, Mr. A. E. Hunt, who 

 was present at the meeting of the British Association at 

 Belfast, has published in your September number a criticism on 

 my address. His paper deals with a variety of matters with which 

 I need not concern myself. His remarks on tourmaline have 

 reference to another author, and not to anything I have written 

 on that subject. He also passes strictures on what he calls 

 "incidental geology," but as he includes Lord Kelvin and Canon 

 Bonney with me in his condemnation, I can only congratulate 

 myself on being in such good company. 



In the following observations I shall confine myself to the 

 criticisms on my Belfast address, and as Mr. Hunt tells us (p. 403) 

 that the whole object of his paper " is to discuss General McMahon's 

 incidental remark — ' the beryl is crowded with liquid and gas 

 cavities, the former containing movable bubbles and deposited 

 crystals as well as water,' " I shall begin by considering the author's 

 remarks under this head. 



Before proceeding further it will be as well to correct an 

 inaccuracy into which the author has fallen. On p. 393 he quotes 

 me as stating that " beryl was the first [mineral] to crystallise, 

 at a temperature approaching 1,200° C." This is a mistake. 

 I certainly said that beryl was the first mineral to crystallise out 

 of the magma, but I nowhere ventured to give the temperature at 

 which that or any other mineral of the granite began to crystallise. 

 I do not think we possess at present sufficient data to enable us to 

 give with any approach to accuracy the temperatures at which the 

 several minerals contained in a granite crystallise, and I did not 

 attempt this task. I was dealing in this portion of my address 

 with contact metamorphism, and my object being to impress upon 

 my hearers the potential energy of a granite in a molten or fluid 

 condition, I thought it desirable to consider " the probable temper- 

 ature reached " by the Satlej granite. 



After considering the melting-points of various granitic minerals, 



and after stating that the case was complicated by the fact that 



1 Bernard, T. : "The Apodidaj," 1892, Macmillan & Co. Quart. Journ. Geol. 

 See, L (1894). 



