94 



University of Calif or>iia. 



[Vol. i. 



were noted, consisting of black bars, or the edges of plates, meeting 

 at angles of 6o° or 120 , and separated by streaks of leucoxene. It 

 is all strongly magnetic, the rocks containing the forms shown in 

 Fig. 8 being capable of being lifted by a weak magnet in fragments 

 from 6 to 7 mm. in diameter. A portion of the rock was powdered, 

 and the opaque grains extracted with a weak horseshoe magnet. 

 These, after being boiled for some time in hydrochloric acid, gave 

 distinct reactions for titanium. A portion of the slide shown in Fig. 

 8 was uncovered, and treated for about one and one-half hours with 

 hot strong hydrochloric acid without dissolving the opaque masses. 

 The only effect of the acid was to reveal a distinct fibrous, or fine 

 platy structure not noticeable in transmitted light, but conspicuous 

 in incident light, and consisting of narrow black strips meeting at 

 angles of 6o° and 120 , and separated by narrow bands of a blue- 

 gray substance. Teall, in his paper on the Whin Sill,* has suggested 

 that magnetite and ilmenite are probably frequently intergrown, and 

 the observations of the writer seem to corroborate this view. Both 

 magnetite and ilmenite are probably present, but there are various 

 intermediate mixtures which cannot be referred definitely to one or 

 the other mineral. The large ragged form shown in Fig. 8 resem- 

 bles very closely some intergrowths of magnetite and ilmenite 

 described and figured by Teall in his paper. 



Apatite can generally be detected in the fresher and more 

 coarsely crystalline slides, and is sometimes quite abundant. It 

 occurs, included in the feldspars and augite, as rather slender color- 

 less prisms, showing the usual blue-gray interference colors, and 

 isotropic hexagonal cross sections of apatite. 



Having described in detail the separate minerals we turn now 

 to a consideration of the special features of particular occurrences 

 of the diabase. On the neck between the lighthouse and the fog 

 siren it is a dark green rather coarsely crystalline rock, with a 

 somewhat peculiar structure. Both on weathered surfaces and 

 fresh fracture it appears as a very coarse felt of long, blade-like 

 crystals, the augite, plagioclase, and iron ores being recognizable 

 with the unaided eye. One feldspar was noted about 2 cm. in 



* Chemical and Microscopical Characters of the Whin Sill, Quart. Jour. 

 Geolog. Soc, Vol. XL, pp. 650-652, PI. XXIX. 



