Professor K. Busz — Granojohyre Dyke in Scotland. 439 



changed into very finely striated and distinctly, though not strongly, 

 pleocbroic steatite of a light-green colour. 



The alteration of both diallage and augite seems to begin with 

 the excretion of round or oval-shaped grains of magnetite or titanic 

 iron-ore, sometimes in such quantities that the augite seems to be 

 nothing but an agglomerate of them ; then there appears on the 

 margin a broader or narrower border of green or brownish-green 

 hornblende, which in the end entirely replaces the augite. This 

 hornblende shows mostly the characteristics of uralite, but there 

 also occasionally occur well-defined crystals which show no signs 

 of the fibrous structure so characteristic of uralite, and to judge 

 from their peculiar brownish-green colour they may perhaps be 

 identical with the variety called barkevikite. The formation of 

 biotite is less common and generally occurs in connection with 

 green hornblende, intergrown with which it forms reddish-brown 

 lamellae or irregular patches. Also the alteration described by 

 Professor Sollas as the breaking up of the crystals into numerous 

 granules has been observed in some cases, always accompanied by 

 the formation of magnetite; in this case a perfect recrystaliization 

 has taken place. 



These are, in short, the results of the examination of the gabbro 

 and the gabbro-porphyry. 



We must now turn to the granophyre. It is a rather fine-grained 

 rock of grey colour. According to its macroscopic appearance it 

 might be termed a microgranite or aplite. It contains numerous 

 black patches of gabbro of various size up to several inches in 

 diameter, and is spotted with minute black specks ; felspar crystals 

 are visible in great numbers, and reach up to 2 mm. diameter. 



Under the microscope the rock presents a somewhat peculiar 

 appearance, due to the remarkable way in which quartz occurs. It 

 forms, as a rule, small crystals, with usually well-defined outlines, 

 whereas the orthoclase appears as the interstitial matter. The quartz 

 crystals are perfectly clear and contain but few inclusions, amongst 

 those many sharply defined, small crystals of zircon, only terminated 

 by the tetragonal pyramid. 



The orthoclase, owing to decomposition, is of a dull grey colour. 

 It not only appears, as mentioned before, filling up the spaces between 

 the quartz crystals, but also forms rectangular crystals up to two 

 millimetres in size ; the greater part of the rock, in fact, is made 

 up of such larger crystals, while quartz and the interstitial ortho- 

 clase form a kind of groundmass. 



The orthoclase crystals exhibit in the centre a fairly fresh appear- 

 ance, and are sometimes even quite colourless, while the outer 

 zone shows exactly the same features as the felspar of the ground- 

 mass. Under crossed nicols this clear centre in many cases is seen 

 to consist of twinned triclinic felspar, showing the same optical 

 properties as that in the gabbro. These parts may therefore 

 undoubtedly be regarded as remains of xenocrysts of the gabbro- 

 plagioclase, which by the effect of the acid magma have on 

 their outer parts been transformed into orthoclase ; this alteration 



