Vol. 50.] OF CARROCK PELL. 331 



newer than the acid intrusion 1 ; but the way in which the gabbro 

 is differentiated affords at least as strong an argument in the same 

 direction. It is noteworthy that, so far as the few observations of 

 specific gravity go, the diabase seems to be much more uniform 

 than the gabbro, as if the requisite condition for differentiation, 

 namely, cool encasing walls, had been wanting in the case of the later 

 intrusion. 



Again, if the basic margin may be taken as marking with 

 tolerable uniformity the original boundary of the gabbro mass, its 

 completeness or otherwise will give information with respect to 

 subsequent accidents which may have affected it. The presence of 

 a basic border on the south side of the intrusion may be taken as 

 indicating that very little of the gabbro is lost in consequence of 

 the bounding fault. On the northern boundary some irregularities 

 occur which suggest that portions of the basic margin of the mass 

 may have been carried away by the later intrusion of granophyre, 

 and I shall have occasion later to notice phenomena in the latter 

 rock which fully accord with this idea. Again, the narrowing of 

 the outcrop of the more acid type of gabbro towards the east seems 

 to point to a termination of the intrusive mass in that direction, so 

 that no fault would be required to account for the non-appearance 

 of the gabbro on the other side of the valley alluvium. As regards 

 the westward termination of the intrusion, the country there is 

 much concealed by peat-mosses, but the thoroughly basic character 

 of the gabbro seen in Eoughten Gill and in Brandy Gill 2 may 

 perhaps point to a coming together of the northern and southern 

 boundaries of the original intrusive body, which is broken into by 

 considerable masses of granophyric rocks. 



7. Reactions between Gabbro and Enclosed Masses of Lava. 



There remain to be briefly noticed certain special modifications of 

 the gabbro which are of an entirely different order from those 

 described above, being local or ' contact '-phenomena. As already 

 mentioned, some of these peculiarities are connected with the enclosed 

 patches of volcanic rocks, others with the proximity of the large 

 granophyre intrusion. Only the former will be treated at length in 

 this place. 



I have stated that the masses of basic lava enveloped by the 

 gabbro are readily identified as members of the Eycott Hill group, 

 the typical locality for which is less than 3 miles distant. The 

 rocks are nevertheless very considerably metamorphosed, and the 

 gabbro in their immediate vicinity shows certain signs of inverse 

 metamorphism. The lavas as they occur at Eycott Hill were briefly 

 described by Mr. Clifton Ward, 3 who gave chemical analyses 



1 I have found no actual exposure of the contact of granophyre and diabase 

 in situ, but junction-specimens of the two rocks occur among the loose blocks 

 to the north of Great Lingy, and in these the diabase takes on a very fine- 

 grained texture. 



2 Specific gravities : Roughten Gill 3'113, Brandy Gill 3'065. 



3 Monthly Microsc. Journ. vol. xvii. (1877) pp. 239-246. 



