322 Scientific Proceedings, Royal Dublin Society. 
out porphyritically. At the actual contact with the schists, the 
groundmass becomes trachytic when viewed in section; but the 
proportionate abundance of the hornblende prisms and the felspar _ 
appears to be the same as in the central mass. The rock, which 
would be styled a porphyrite by many in the field, may be placed 
with Rosenbusch’s vogesites, rather than with the compact syenites, 
on account of its richness in amphibole. It is, moreover, in all 
probability, near the basic end of the vogesite series, and must be 
regarded as indicating a passage from that series to the campton- 
ites. . 
It would be of considerable interest if we could trace these 
lamprophyric dykes down to the caldron from which they have 
arisen. The view at present favoured is that such rocks have 
come up as products of differentiation from a magma of more 
basic character, and their poverty in silica, as compared with their 
richness in alkalies, is one of the main points that distinguish 
them from the contents of ordinary igneous caldrons in the 
crust. Prof. W. C. Brogger’s admirable work on the basic 
eruptive rocks of Gran’ met with a somewhat natural criticism - 
from Prof. Johnston-Lavis,? who pointed out that the rock-types 
alleged to have been produced by differentiation might have arisen 
from the intermingling of a normal magma with matter derived 
from the sedimentary or other masses with which it came in 
contact. The camptonitic zones formed round inclusions of sedi- 
mentary rocks immersed in a “ bostonite” magma’ certainly 
appeared to lend themselves to such an explanation; but Prof. 
Brogger has more than once‘ reaffirmed and strengthened his 
position, refusing, at the same time, any assistance from the 
theories as to the potency of contact-absorption, which have been 
developed by the observations of M. Michel Lévy and other 
writers. When Messrs. M‘Henry and Watts’ suggested that the 
“micaceous felsites’’ of the Barnesmore area in southern Donegal 
1 Quart. Journ. Geol. Soc. London, vol. L., (1894), p. 15. 
2 Geol. Mag., 1894, p. 252. 
3 Quart. Journ. Geol. Soc., vol. ., p. 25. 
4“ Die Gesteine der Grorudit-Tinguait-Serie’’ (1894), p. 158; and, in some detail, 
‘Das Ganggefolge des Laurdalits’’ (1898), p. 347, also, in general, pp. 334-351. 
5 «Guide to Collection of Rocks and Fossils,’’ Geol. Surv. Ireland (1895), 
Dena 
