242 Cotre—On the Geology of Slieve Gallon, in the County of Londonderry. 
of rocks by a process of absorption, in some cases so complete as to resemble 
solution. 
(i1.) It cannot do this unless space is made for the product of mingling, which, 
of course, need not equal in volume the united volumes of the invading and 
absorbed materials. Provision for this is very often made from the outset by the 
formation of a dome in the superincumbent rocks, the earth-pressures that give 
rise to this dome being usually responsible for the simultaneous flow of molten 
matter into it. 
(ii.) The product of mingling may finally begin to erystallise, and the 
‘‘ plutonic rock”? in due time presented to us is thus chemically different from 
that which the invading magma alone would have produced. It is also likely to 
be more complex in its mineral constitution. 
(iv.) But, during consolidation, differentiation, from various causes, may be 
set up in the mingled magma, and the whole of the absorbed material may, in 
exceptional circumstances, become drained away, and may disappear into the 
depths, leaving a rock as pure, or almost as pure, as that which the invading 
magma would have formed before contamination. 
(v.) Hence a plutonic mass may be a pseudomorph of the rock invaded by 
it, progressing inch by inch so long as it remains molten. It may be as truly a 
pseudomorph as flint is of massive limestone ; but it seems improbable that original 
structures would, in such extreme cases, be preserved. In ordinary cases, flakes 
of the pre-existing rock may remain, and may mark out the old structure; the 
masses cited by Mr. Hugh Miller are possibly of this description. 
(vi.) A plutonic rock may thus appear to have arisen as the product of meta- 
morphism of a series of rocks 7 situ, in which case the plutonic mass seems actually 
to occupy the space once filled by a portion of that series. Where, indeed, the 
pre-existing rocks are not arched over the plutonic mass, either metamorphism or 
absorption seems the only way out of the difficulty. The older geologists set 
aside the chemical difficulties in such cases as Slieve Gallion, and urged metamor- 
phism én sifu as the cause of the plutonic core. I have tried to show above, that 
absorption is possible, but may be accompanied by transference of much of the 
absorbed material. ‘The final igneous mass, on consolidation, may therefore be a 
true pseudomorph, and not a paramorph, of the rocks replaced by it. 
If the foregoing considerations are accepted as a contribution to the discus- 
sion of Messrs. Horne and Greenly’s remarkable Paper on the foliated granites of 
Sutherland,* it is all that I can fairly ask. They arise, however, naturally 
out of the problems presented by Slieve Gallion. 
* Op. cit., Discussion, pp. 648-650. 
