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such sections show in convergent light the bisectrices and the optic axes. The 

 crystals are frequently twinned in such a way that the orthopinacoid (100) is 

 both twin-plane and face of composition ; and not seldom several lamellae are 

 seen to be interposed between the two principal portions. We have no analyses 

 of the hornblende of the British basic eruptive rocks. It occurs in. a doleritc 

 from Necropolis Hill, Glasgow (see fig. 1, plate XIII.), in certain allied rocks 

 from the neighbourhood of Plymouth, and in the basic diorites of the 

 Warwickshire type. (1) In the last mentioned rocks it is sometimes found as 

 ophitic plates and sometimes as well formed crystals. Brown hornblende and 

 augite are sometimes found intergrown with each other in such a way that the 

 plane of symmetry and vertical axis are common to both minerals. This may be 

 observed in the Whin Sill and in a " greenstone " from St. Minver, in 

 Cornwall. The relations are similar to those described in the case of the 

 Inchcolm picrite. The development of secondary hornblende as a consequence 

 of the alteration of pyroxene has already been referred to. Uralite, 

 actinolite, and " compact " hornblende may be produced in this way, and the 

 latter variety may be either brown, green, or colourless. Brown hornblende 

 of a secondary character appears however to be comparatively rare. 



Magnetite find Ilmenite. Oxides of iron with a variable amount of titanic 

 acid are almost constantly found in basic eruptive rocks. They occur as 

 crystals, skeletons, and grains. In thin sections they are perfectly opaque. 

 Magnetite crystallises in the form of the octahedron, and the crystals are often 

 very minute. Sections may be square, triangular, or hexagonal. Twins 

 according to the spinelle-type, that is with an octahedral face as the twinning 

 plane, are common, and sections of such twins taken at right angles to the 

 twinning plane arc often somewhat lath-shaped in section with a re-entering 

 angle at one end and a projecting angle at the other. The lath-shaped form 

 is due to a flattening in the direction of the twinning-axis. In rocks which 

 contain a glassy base minute crystals and grains of magnetite are often seen to 

 be arranged in straight lines so as to form rods, which in turn build up 

 skeleton crystals. An exquisite illustration of this is seen in a basalt from 

 North Berwick (Scotland) figured by VOGELSANG. (2) Magnetite may be 

 entirely removed from a thin section by treatment with hot hydrochloric acid, 

 and this is sometimes a useful means of distinguishing it from ilmenite ; the 

 latter mineral being much less readily affected. The mineral may be easily 

 extracted from the powder of the rock with a magnetic needle. The 

 composition of pure magnetite is represented by the formula FeO Fe 2 3 ; 

 that of the basic igneous rocks usually contains a considerable amount of 

 titanic acid. Magnetites which are poor in titanic acid yield hydrated ferric 

 oxide when subjected to alteration by surface agencies ; those rich in titanic 

 acid decompose in the same way as ilmenite. 



Ilmenite occurs in irregular masses without definite crystallographic 

 boundaries and as hexagonal tables. Beautiful examples of the latter 

 mode of occurrence may sometimes be observed in the dolerite of Rowley 



(1) See ALLI-OKT. Q.J.G.S., XXXV., 1879, p. 637. 



(2) Die Krystalliten. fig. 1, Taf. XIII. 



