318 MB. ALFRED HAB.KEK ON THE GABBRO [Aug. 1 894, 



in some slides. These occur more plentifully in some intrusions 

 near Haweswater, such as that which forms Walla Crag. Of olivine 

 I have found no certain trace, even in the most highly basic variety 

 of the gabbro. This mineral seems to be unknown in the Lake 

 District, except in a few minor intrusions such as that of Little 

 Knott. 



The presence of quartz in the Carrock Pell gabbro has long been 

 known. It occurs sometimes in interstitial grains, but more 

 frequently as a constituent of micropegmatite filling the interspaces 

 between the augite and felspar-crystals. The intergrown felspar is 

 probably in part orthoclase, the analysis showing a certain amount 

 of potash in the rocks. The micropegmatite is a prominent con- 

 stituent in the most acid gabbro, and is tolerably plentiful in many 

 examples in which the silica-percentage must be quite low, failing 

 completely only in the very basic varieties. (See PI. XVII. fig. 3.) 



Apatite, in rather stout prisms, is capriciously distributed. In 

 many slides it is wanting, but it occurs in both acid and basic 

 varieties of the gabbro, and in the latter sometimes rather abun- 

 dantly. Grains of sphene are seen in some slides, but their form 

 and their association with the iron ores are such as to suggest a 

 secondary origin for the mineral. 



Opaque iron-ores are very sparingly present in the most acid 

 variety of the rock, but become increasingly abundant in the more 

 basic examples, and in some form a very important part of the mass. 

 In one case nearly 25 per cent, of the finely powdered rock was 

 extracted by a horseshoe magnet. These rocks, very rich in iron 

 ores, strongly attract the magnetic needle, but they show no evident 

 polarity and do not orient themselves when freely suspended. The 

 iron ores are in the main among the earliest products of crystalliza- 

 tion in the gabbro, but they do not, as a rule, show any perfection 

 of crystal outline. Viewed in reflected light some of the crystal- 

 grains show a black or bluish-black colour, while others have a tinge 

 of grey. The former only are attacked by cold hydrochloric acid. 

 It appears, therefore, that we have both magnetite and ilmenite 

 present, and the chemical analyses given below confirm this con- 

 clusion. In some places a crystal-grain consists partly of one, partly 

 of the other mineral, with crystal-faces common to the two and a 

 dividing line like a twin-line parallel to a crystal-boundary. (See 

 PL XVII. fig. 5.) This is an arrangement which does not seem to 

 be common in other rocks. Further, there are not wanting here 

 indications of a more minute intergrowth of the two iron-ore 

 minerals, and the colour of some grains leaves doubt whether they 

 should be referred to magnetite or ilmenite. It is worthy of notice, 

 too, that apparently almost the whole of the iron ore in the rock 

 is magnetic. More precise knowledge concerning ' titaniferous 

 magnetite ' and ' titanomagnetite ' seems desirable. 



Pyrites is only locally present in the very basic gabbros (Arm o' 

 Grain, etc.). 



