Vol. 51.] GRANOPHYRE AND THE GRAINSGILL GREISEN. 137 



granophyre, and they frequently show a clear border surrounding a 

 turbid nucleus of rounded shape and corroded appearance. Of such 

 a derived origin there seems to be clearer indication in the case of 

 the less fusible minerals, the iron ores and apatite. The shape of 

 some of the larger opaque grains in such rocks as Nos. 2 and 3, 

 described above, is significant. Especially may be remarked skele- 

 tons or frameworks, apparently of ilmenite, which perhaps represent 

 the intergrowths of ilmenite and magnetite in the gabbro from 

 which the latter mineral has been dissolved out. The apatite- 

 crystals in the basic modification of the granophyre exactly resemble 

 those in the gabbro, and are probably derived, without alteration, 

 from that source. It will be remembered that this mineral is 

 foreign to the normal granophyre. 



The ' caustic ' action of a fluid magma upon solid rocks has been 

 studied by many geologists in connexion with the phenomena of 

 enclosed fragments, and an excellent summary of knowledge on 

 this point has recently been given by Zirkel. 1 From this it appears 

 that in extreme cases an enclosed fragment of an igneous rock may 

 be completely dissolved, with the exception of a few refractory 

 minerals such as zircon, sappbire, etc. The corrosive action is most 

 intense when the rock attacked and the absorbing magma differ 

 widely in acidity ; and we thus see why the action of the granophyre- 

 magma upon the highly basic gabbro was much more energetic tbau 

 that of the gabbro-magma upon the enclosed masses of basic lava 

 (compare my former paper). A high temperature is another factor 

 involved ; and probably the peculiarity of the conditions in the 

 present case lay in the intrusion of a considerable body of highly 

 heated acid magma into contact with an extremely basic rock which 

 had not had time to cool. It is in accord with this idea that we 

 find no phenomena like those under discussion in the smaller 

 granophyre-intrusions of the district, including the dykes and veins, 

 which are of uniformly acid character. We have already seen 

 evidence that the granophyre-magma was intruded before the 

 beginning of crystallization in it, and therefore in a highly heated 

 condition. On the other hand, it is probable that its temperature 

 was not very much higher than the minimum thus implied, for the 

 fact that the dissolved basic material was not diffused through the 

 whole magma, but remained near the margin, seems to indicate a 

 certain degree of viscosity. 



The only instance that I have found, of an unusually dense and 

 presumably basic variety of the granophyre in the interior of the 

 mass, is in the face of the Scurth opposite the ' Apronful of Stones ' 

 marked on the 6-inch map. The specific gravity here was found 

 to be 2-814. This may possibly be due to the absorption of an 

 enclosed portion of Eycott lava. Near the same spot I collected 

 specimens which seemed in the field to represent such an inclusion 

 in a highly altered condition, but, if this be their true nature, the 

 metamorphism is too great to admit of any confident identification. 



3 ' Lehrbuch der Petrographie,' 2nd ed. vol. i. (1893) pp. 593-602. 



