262 



and containing inclusions of felspar. The pleochroism in sections at right 

 angles to the principal cleavage is generally very strong. The colour 

 changes from a pale yellow or yellowish brown for rays vibrating at right 

 angles to the principal cleavage (parallel to a) to a very dark brown and 

 in some cases almost black for rays vibrating parallel to the principal 

 cleavage. There is, in the nearly uniaxial biotites, no appreciable difference 

 for rays vibrating parallel to the /3 and 7 axes, both of which lie approxi- 

 mately in the plane of easy cleavage. The biotite very frequently contains 

 inclusions of apatite and magnetite. By alteration it passes into chlorite 

 and this alteration often proceeds along the cleavage planes so that 

 lamellae of chlorite alternate with lamellae of unaltered biotite. Minute 

 granules and prisms of epidote are often associated with the chlorite. The 

 ultimate alteration products are carbonates and limonite. 



Iron-ores. These occur sparingly in rocks of intermediate compo- 

 sition. In the andesites they are present as grains and crystals in the 

 ground-mass and as inclusions in the porphyritic constituents. We have 

 already explained that they are in part of secondary origin, arising from 

 the action of the magma on crystals of hornblende and biotite. In the 

 diorites they are also present in grains and crystals (often octahedra ol 

 magnetite), usually larger than in the andesites. As a rule the crystals 

 appear to be magnetite and not ilmenite. The titanic acid of the diorites 

 appears to occur in sphene rather than in iron-ores. In some cases it 

 occurs in the form of rutile. Thus CROSS W observed rutile-inclusions in 

 the green biotite of a quartz-diorite from St. Brieux in Brittany. It is 

 possible, of course, that in this case the rutile is a secondary mineral 

 arising in consequence of the separation of titanic acid from the biotite. 

 The colour of the biotite, as ROSENBUSCH points out, is suggestive of 

 alteration. 



Si^hene. This mineral is extremely common in the diorites, especially 

 in the hornblende-diorites. It occurs in well-formed crystals and as 

 irregular grains. The crystals are generally yellowish or reddish; the 

 minute grains are often colourless and similar to those derived from the 

 alteration of titaniferous iron-ore. One very characteristic section is the 

 acute rhombic. Twinning is not uncommon and the trace of the face of 

 composition bisects the acute angles in the rhombic sections. The mineral 

 possesses very high refractive and double refractive power, and these are 

 the most useful characters in recognizing the mineral in the absence of 

 form. The refractive power is so high that the mineral always appears 

 bounded by broad dark borders. Another feature of considerable use is 

 the strong dispersion of the optic axes (p > v). This may be determined 

 in the minute grains which arise in consequence of the alteration of 

 titaniferous iron-ores. 



Apatite. This mineral is almost constantly present but it presents no 

 features worthy of special note. Zircon occurs occasionally as an accessory 

 constituent. 



(1) T.M.M., 1880, III., p. 369. 



