346 LIEUT.-GEN. C. A. M c MAHON ON ROCKS OF [Aug. 1894, 



the extinction is 15° from the plane of twinning in both sets of 

 polysynthetic twins, or straight in one set and approximately 15° 

 in the other set of twins. This is very characteristic of all the 

 orthoelases in the lavas in this vicinity, and shows that intergrowths 

 of microcline with orthoclase are not uncommon. The cross hatch- 

 ing, so commonly seen in the microcline of granites, has not been 

 observed in these rocks. In many cases the felspars are idiomorphic. 



Nos. 19 and 20 contain grains of free quartz in the groundmass, 

 and these possess liquid inclusions with bubbles. 



All the slices contain matted fibrous masses, and more or less 

 radiating tufts, of fine needle-like prisms of actinolite. In habit it 

 is like the anthophyllite of No. 8 ; in these slices it varies in 

 colour from a pale-greenish to a brownish-yellow. Here and there 

 it is slightly dichroic. The angle of extinction varies from 0° to 

 16° ; the major axis of elasticity is at a high angle to the length 

 of the prisms ; and the mineral is not acted on by prolonged 

 heating in concentrated hydrochloric acid. 



The secondary origin of the actinolite is evident from the fact 

 that it extends, in some cases, from the body of the rock into the 

 quartz-veins that traverse some of the slices. This is an interesting 

 fact, as it shows that the genesis of the actinolite was contemporaneous 

 with the formation of the quartz-veins. As the quartz in these veins 

 contains liquid cavities and bubbles, I think it probable that the 

 actinolite and the quartz are both due to the contact-action of the 

 neighbouring Dartmoor granite. In No. 20 the actinolite is of dis- 

 tinctly yellow tint and seems to be on the road to conversion into 

 epidote. In double refraction, however, it is far from this mineral. 



All the slices contain more or less magnetite, ilmenite, or ferrite. 



No. 21 contains some lacunas stopped with a chloritic-serpent- 

 inous mineral associated with quartz. No. 22 contains apatite, 

 No. 19 a little haematite, and a red to green mica in leaves and 

 radiating fibres. Digestion in hot concentrated hydrochloric acid 

 removes much of the colouring matter, but does not dissolve the mica. 



No. 23 is darker in colour than the specimens described under 

 the head of ' trachytes,' beiDg a light greenish-grey on the freshly- 

 fractured surface, and rusty-looking on its weathered face. Decom- 

 position haB advanced so far in this specimen that I find it 

 impossible to state definitely what its original structure was. It 

 seems to have been a lava, and that is all I can say. 



Remains of what appear to have been felspar-microlites and fine 

 lath-shaped prisms of that mineral can be made out, but they are 

 highly altered ; they have straight extinction. The slice is 

 profusely dappled with leucoxene after ilmenite, the inclusions in 

 the original ilmenite remaining. There are also a few specks of 

 magnetite or ilmenite. 



The slice contains an abundance of calcite, in tabular twinned 

 crystals and dotted all over the field in minute granules. A pale 

 yellowish- green to almost colourless, serpen tinous-looking, isotropic, 

 and structureless substance occurs in patches and in lacunas. 



