336 MK. ALFRED HARKER ON THE GABBKO [Aug. 1894^ 



Fig. 2. [1867]. Gabbro, Iron Crags, about 200 yards W.N.W. of tbe sheep- 

 fold. This is drawn with polarized light (crossed nicols) to show 

 what seems to be secondary twin-lamellation on both the albite- and 

 the pericline-law in the plagioclase. In one part of the crystal the 

 pericline-twinning affects only alternate albite-lamellse. Other 

 crystals in this slide show albite-lamellation in evident relation to the 

 strain attending flexure. The dark line is a crack in the slice. See 

 p. 316. 



Fig. 3. [1874]. Gabbro, top of White Crags. The figure shows idiomorphic 

 crystals of plagioclase moulded by a shapeless plate of more uniformly 

 turbid, untwinned orthoclase, well seen in the upper part of the 

 drawing. Lower, and to the left, is an interstitial patch of micro- 

 pegmatite, in which the felspathic constituent is probably also ortho- 

 clase. Pyroxenes and iron ores do not appear in the portion of the 

 slice figured. See p. 318. 



Fig. 4. [2046]. Metamorphosed gabbro, Brandy Gill, 50 yards north of the 

 upper ' Bield.' This is a very basic marginal variety of the rock, un- 

 usually rich in prisms of apatite, which are seen in abundance. The 

 rock is profoundly modified by thermal metamorphism, the pyroxene 

 being wholly transformed, partly into green actinolitic hornblende, 

 partly into matted patches of brown mica-scales. The latter mineral 

 occurs characteristically in the neighbourhood of the grains of iron ore, 

 from which it has probably taken up some ferrous oxide and titanic 

 acid. The clear grains in the lower right-hand part of the figure are 

 portions of one crystal of felspar, divided by narrow veins now con- 

 sisting of brown mica. The clearness of the felspar seems to be a 

 characteristic of the metamorphosed gabbros. See p. 334. 



Fig. 5. [1526 and 1866]. Grains of iron ores in the gabbro. The example on 

 the right, from the upper part of Furthergili Sike, shows irregular 

 patches of magnetite (black) and ilmenite (grey). That on tbe left, 

 from near the top of the same sike, shows the two minerals forming 

 parts of one idiomorphic crystal, the dividing line being parallel to a 

 crystal-boundary. See p. 318. 



Fig. 6. [1622J. Modified Gabbro, crags in upper part of Furthergili Sike. 

 This is from the actual margin of the gabbro, and is a highly basic 

 variety, of specific gravity 3'122, rich in apatite (see upper part of 

 figure). Nevertheless it contains quartz and micropegmatite (lower 

 part of figure). This is probably due to an injection of the already 

 consolidated gabbro by the later granophyre-magma, in the manner 

 described by Prof. Sollas. At Carrock Fell this action seems to be 

 exceptional, and is confined to the immediate contact of the two 

 intrusions. Other parts of this slide and other specimens of the 

 same rock show various phenomena of thermal metamorphism in the 

 gabbro. See p. 335. 



Discussion. 



Mr. Harr believed that the age of the gabbro intrusion had yet 

 to be determined. The Author's work had for ever set at rest the 

 idea that the gabbro had been formed by alteration of the volcanic 

 rocks of the Eycott group ; for the proofs of intrusion of the gabbro 

 into these were complete. The mode of occurrence of the gabbro 

 and granophyre reminded him of the description of masses of these 

 rocks in Scotland, about which the Society had recently heard much. 

 These were points of local interest, but the main object of the paper 

 was to describe the variation in the different parts of the gabbro 

 mass, and from what he had seen of the district he believed that 

 the Author had established his points. 



Prof. Judd congratulated the Author of the paper on taking up thi& 



