438 Professor K. Busz — Omnophyre Dyke in Scotland. 



individuals are in so far somewhat different as they exhibit a more 

 lath-shaped form. 



In other parts rhombic pyroxene appears in considerable quantity, 

 and generally in large crystals, showing the characteristic pleo- 

 chroism of hyperstJiene — pale green, yellow, pink — and the parallel 

 extinction ; the individuals are not homogeneous, but show a poikilitic 

 structure, being intergrown with numerous, irregularly disseminated, 

 lath-shaped, plagioclase crystals. 



The microscopic examination of the porphyritic plagioclase crystals 

 shows that they are also nearly pure anorthite, like the plagioclase 

 in the groundmass and in the above-described beerbachite, the 

 maximum extinction angle being about 34° on the basal plane. The 

 black colour is due to a great quantity of black dust-like inclusions 

 (similar to the plagioclase in some diorites from Sweden, commonly 

 known as black Swedish granite). They are in some parts of the 

 crystals more densely distributed than in others, and so give them 

 a cloudy appearance. 



We have now to consider how these constituents of the gabbro 

 have been altered by the intrusion of the granophyre dyke. The 

 latter, of a grey colour, is on an average one foot wide, and has 

 been almost vertically intruded into the surrounding gabbro. It 

 contains a great number of xenoliths, which belong to the fine- 

 grained gabbro as well as to the porphyritic variety. A detailed 

 description of the granophyre will follow later on; for the present 

 it may suffice to say that the acid magma has absorbed a great 

 quantity of the gabbro, and the xenoliths can be studied in every 

 stage of absorption. 



The alteration of the plagioclase can be studied best on the 

 xenoliths of the gabbro-porphyry. On the margin towards the 

 granophyre they first lose their transparency, and the outer zone 

 becomes a dull greyish-brown similar to decomposed orthoclase irt 

 granite ; the examination by crossed nicols shows that actually 

 a gradual alteration into orthoclase has taken place, the zones being 

 like those in the Bytownite from Barnavave, not sharply defined, but 

 gradually passing into one another. The centre of the crystals does 

 not show much of this alteration, there being only more or less thin 

 threads of the brownish substance. Further away from the line of 

 junction, where the granophyre meets the gabbro, the plagioclase 

 hardly shows any alteration at all. 



Much more conspicuous are the alterations which have taken place- 

 in the augite — a fact quite in accordance with the observations 

 made by Professor Sollas and Mr. Harker. Here, as in the 

 Barnavave gabbro, there are principally three different products 

 of alteration, those being green, rarely brownish-green hornblende, 

 biotite, and granular augite, to which in some cases steatite may be 

 added. The last-named occurs only in the gabbro-porphyry, and is 

 the alteration product of the rhombic pyroxene. Near the junction 

 line of the two rocks the outer zone of the hypersthene assumes 

 a green colour and becomes less transparent; this alteration increases 

 towards the granophyre until the entire hypersthene substance is 



