96 
SUBTERRANEAN VOLCANIC ACTION 
BOOK I 
fields of Britain, where many examples of this change have been noted, the 
igneous material is known as “ white trap.” The iron oxides have been in 
great part removed, or, together with the lime of the component minerals, 
have been converted into carbonates. Traces of the original felspar crystals 
may still be detected, but the grouudmass has been changed into a dull, 
earthy, friable and decomposed substance. 
Nearly always, however, the alteration of the intrusive magma has 
resulted from the incorporation of portions of the surrounding rocks. 
Ileference has been made above to the alteration of the Carrock Fell gabbro 
by the absorption of some of the basic lavas around it. But still more 
remarkable is the change produced in some acid rocks by the incorporation 
of basic material into their substance. Professor Sollas has described in 
great detail a remarkable instance of this effect in the probably Tertiary 
eruptive rocks of the Carlingford district in the north-east of Ireland. 
He has ascertained that the eruptive gabbro of that district is older 
than the granite, for it is traversed by granophyre dykes which enclose 
pieces of it. The granophyre dykes, on the other hand, often show a 
lithoidal or chilled margin, which is not visible in the gabbro. He believes 
that the gabbro is not only older than the acid protrusions, but was 
already completely solid, traversed by contraction-joints, and probably frac- 
tured by earth-movements, before the injection of the granophyric material, 
which at the time of its intrusion was in a state of extreme fluidity, lor it 
has found its way into the minutest crael^s and crevices. He has especi- 
ally studied the alteration produced by the granophyre upon the enclosed 
pieces of basic rock. The diallage, isolated from the other constituents 
of the gabbro, may commonly he seen to have broken up into numerous 
granules, like the augite grains of basalt, while in some cases biotite and 
hornblende have been developed with the concomitant excretion of mag- 
netite. Tlie acid rock itself has undergone considerable modification owing 
to the incorporation of basic material into its substance. Professor Sollas 
distinguishes the following varieties of the rock : — Biotite-granophyre, biotite- 
amphibole-granophyre, augite-granophyre, diallage-amphibole-augite-grano- 
phyre.^ 
Similar phenomena have been described by Mr. Harker as occurring 
where granophyre has invaded the gabbro of Carrock lell.- The same 
observer has more recently detected some interestuig examples furnished 
by injections of Tertiary granophyre in the agglomerates of Skye. The 
acid rock is roughly estimated by him to have taken up aljout one-fourth of 
its bulk of gabbro fragments. He has investigated the minute structure of 
the rock thus constituted, and has been able to recognize the aiigite of the 
original gabbro, in various stages of alteration and completely isolated, the 
other minerals having been dissolved in the acid magma. 
' Trans. Iloy. Irish Acad. xxx. (1894), part xii. p. 477. 
- Quart. Joxirn. <Icol. Soc. li. (1895), p. 183. 
Op. cit. lii. The metamorphisni produced upon fragments ol difiereiit kinds of foreign 
material enelosed within various igneous rocks has in recent years been studied in great detail by 
Professor Lacroix — Les Enclaves dcs Jioches Volcaniques, Macon, 1893. 
