Vol. 51.] GRANOPHYRE AND THE GRAINSGILL GREISEN". 131 



considerably less acid than what has been described as the normal 

 type. The specific gravity increases from 2*7 to 2 - 8 and even to 

 2*9 or more. This is at the southern margin of the intrusion, 

 where the silica-percentage of the granophyre, in contact with the 

 gabbro, has fallen to less than 60. The microscope shows that 

 concurrently the rock becomes richer in augite, and in its most basic 

 phase contains a notable proportion of iron ores and apatite, with 

 sometimes a little biotite. 



These variations are sufficiently apparent in hand-specimens taken 

 in the field. The normal type of granophyre shows small scattered 

 crystals of black augite and white or glassy-looking oligoclase,in afine- 

 textured grey, or cream-coloured, or reddish groundmas3. In some 

 of the more acid examples the augite is wholly or almost wholly absent, 

 and the rock has a white colour. This is the case at the head of Brandy 

 Grill and in the peat-moss south of Drygill Head, and the specific 

 gravity of these specimens is naturally very low (2-578 and 2-530). 

 In other places the granophyre becomes richer in augite, the lustrous 

 columnar crystals of that mineral and the duller black spots which 

 mark its decomposition-products, together with the white crystals 

 of felspar, being more closely set in the reddish or brownish grano- 

 phyric matrix. This variety prevails especially towards the southern 

 boundary of the Carrock Fell intrusion. The augite is not only 

 more abundant, but in larger crystals, and indeed the whole rock 

 assumes a coarser aspect in this part of the mass. On the actual 

 margin these peculiarities are more marked, and are worthy of more 

 detailed notice. 



Before describing the special phenomena at the junction of the 

 granophyre with the gabbro, however, it will be convenient to 

 enquire whether the facts already broadly stated lead to any plaus- 

 ible explanation of the variation observed in the different parts of 

 the acid intrusion. The occurrence of the densest and most basic 

 type of granophyre on the southern edge of the mass, which seems 

 to represent its base as originally intruded, 1 suggests at once the 

 differentiating action of gravity. There are two ways in which 

 this might conceivably take effect : namely, by a stratified arrange- 

 ment of layers of different density in a magma still wholly fluid, 

 or by the sinking of crystals already separated out in the magma. 



The former hypothesis, which is as old as Durocher's famous 

 theory, has been made the basis of speculations by several writers, 

 and I have endeavoured to develop it in connexion with the suc- 



(iv) Carrock Fell summit : Silica 69044 (Hughes, cit. Ward) ; sp. gr. of 



specimen from same locality, 2-657. 

 (v) Close to junction with gabbro, lower part of Furthergill Sike: 



Silica 60-0 (L) ; sp. gr. 2-805. 

 (vi) Another specimen from the same place : Silica 58'26 (S). 



I have made specific-gravity determinations on specimens from twenty other 

 localities, and found them a useful check upon the examination of hand-specimens 

 and slices. The figures are all corrected to the standard temperature, 4° C. 



1 See my former paper, Quart. Journ. Geol. Soc. vol. 1. (1894), section on 

 p. 314. 



