136 MB. ALFRED HARKER ON THE CARROCK PELL [May 1S95, 



explained by its tendency to weathering ; as seen in Furthergill it 

 is usually very rotten. Dark brown mica, magnetite, and needles 

 of apatite are conspicuous among the partly decomposed augite, 

 hornblende, etc., of the crumbling rock. 



All these facts are consistent with, and seem to lead to, the 

 conclusion that portions of the highly basic gabbro have become 

 incorporated with the acid magma near the junction of the two 

 intrusions. The manner in which the actual margin of the gabbro 

 as now seen is locally impregnated with granophyre proves that 

 such an absorption of gabbro-material into the acid magma must 

 have taken place to some extent, the mode of occurrence of the 

 veins and nests of micropegmatite in the iron-ore-gabbro pointing 

 clearly to corrosive action rather than merely mechanical disruption. 

 The absorption in this way of some constituents of the gabbro must, 

 on the edge of the solid mass, have disengaged crystals of other 

 constituents, to be floated off in the acid magma and, whether 

 fused or not, to modify the composition of the resulting rock. If 

 we may suppose such an action to have taken place to any con- 

 siderable extent, the peculiarities observed in the marginal part of 

 the granophyre find a sufficiently simple explanation. That con- 

 siderable portions have been removed from some parts of the gabbro- 

 margin we have seen independent reasons for believing. (See 

 p. 331 of my former paper, Quart. Journ. Geol. Soc. vol. 1. 1891, 

 and the accompanying map, pi. xvi.) 



If such a reaction as that postulated on the above grounds, 

 between the already consolidated basic gabbro and the acid magma, 

 be admitted as a possibility, it may throw some light on the nature 

 of the process to enquire what has become of the several consti- 

 tuents of the destroyed gabbro. The order of formation of minerals 

 from a rock-magma does not in general correspond exactly with the 

 (reversed) order of fusibility of those minerals, and many geologists 

 have preferred the analogy of the crystallization of a mixture of salts 

 from solution. Conversely, the absorption of crystallized minerals 

 by a fluid magma may be compared with solution or chemical cor- 

 rosion rather than with dry fusion. Since, however, the order of 

 consolidation of the constituents of our gabbro is the same as their 

 (reversed) order of fusibility, this question need not be discussed. 

 The augite of the gabbro, then, would be the mineral most readily 

 attacked, and it seems to have been wholly dissolved and its sub- 

 stance incorporated in thegranophyre-magma ; no augite resembling 

 it is found in the basic margin of the granophyre-intrusion. From 

 its interstitial mode of occurrence in the gabbro it is clear that the 

 removal of the augite from the marginal part of that rock would 

 set free the crystals of the other constituents. The felspar seems 

 also to have been dissolved in great part, though it is possible that 

 some of the relatively large crystals of plagioclase seen in the 

 modified granophyre have been directly derived from the older rock. 

 These crystals are much more plentiful here than in the normal 



