ASSOCIATED ROCKS OF THE LIZARD DISTRICT. 909 



the stage the unaltered part of the crystal becomes (with crossed 

 prisms) as nearly as possible black after it has been moved through 

 29|° from parallelism with a vertical line ; but the other part offers 

 no approach to complete darkness, and, though darkest when near 

 the above position, is not all uniformly at its darkest then. This, 

 however, may be only due to imperfect cohesion of the cleavage- 

 planes. 



Slide IV. is cut from the green-coloured variety of the older 

 gabbro. The principal differences of this from I. and II. are : — that 

 the conversion of the olivine is about as complete as in III., but is 

 unattended by any red colour ; the serpentine is'of a pale yellowish- 

 green tint ; the dark dust is formed in the cracks of the olivine, as 

 described above, but appears to recombine, or in some way or other 

 is made partially to disappear at a further stage of the process ; for 

 some of the most completely changed greenish grains are very fairly 

 clear. The augite or diallage is scarce, and, as a rule, occurs in very 

 small grains. The resemblance of this rock, macroscopic and micro- 

 scopic, to the troktolite of Yolpersdorf is very remarkable, the only 

 difference being that the felspar in the latter shows brighter colours 

 and the serpen tinous part is a shade darker in its green. The 

 cracks traversing the felspar in all these rocks are noteworthy, as 

 they appear to radiate from the serpentinized olivine granules and 

 to imply some strains caused by them in the process of alteration. 

 I may also call attention to the fact that the pyroxenic consti- 

 tuent seldom, if ever, shows any signs of being converted into 

 serpentine. 



The Neiver Gabbro. — This is generally much coarser in texture 

 than the others, and more decomposed. It pierces the adjacent rock 

 with very irregular veins, which sometimes thin away to mere 

 strings not half an inch thick, but are fairly coarse to the last. The 

 plagioclase, often in crystals nearly an inch across, varies from a 

 dull purplish tint to white. The diallage crystals are frequently 

 from | to | inch across. In places red spots may be observed ; these 

 are evidently altered olivine, as in the last rock, but they are often 

 larger, more friable, and irregular in their occurrence. Included 

 fragments of serpentine are also seen. Spots of the green horn- 

 blendic mineral are frequent towards the exterior. As might be 

 expected the weathered surface of the coarser varieties is much 

 rougher than in the older gabbro. The latter rock also has a perfectly 

 close junction with the serpentine, as if it were welded to it ; while 

 between that and the newer gabbro is more or less of a crevice. 



The result of microscopic examination of a slide of one of the 

 dark purple and green specimens is interesting. It contains 

 abundant plagioclase, apparently labradorite, resembling that in the 

 other gabbros, in fair condition but dull in colour, which here and 

 there seems partially replaced by a clear isotropic pseudomorph. 

 The green spots prove to be altered diallage : here and there small por- 

 tions of the original mineral with characteristic cleavage &c. remain, 

 but the greater part is converted into a mass of pale green actino- 

 lite, the crystals being crowded together like interlacing leaves, and 



