Tol. 52.] ROCKS OF THE LIZARD DISTRICT. 39 



Afterwards I described the microscopic structure of the rocks, 

 pointing out that the serpentine is an unusually compact variety, and 

 .stating that I regarded the mass as representing a block of horn- 

 blende-schist, caught up by the serpentine, altered thereby, and 

 probably since then further changed by the action of water. In 

 the 18 years which have elapsed since I wrote these descriptions, 

 my experience has been considerably enlarged, yet I still find the 

 rock numbered (4) a very perplexing affair. I note that it is 

 .sporadically porphyritic, that the larger crystals under the micro- 

 scope appear to exhibit traces of ' lustre -mottling/ that the 

 wedge-shaped piece included in the serpentine (5) is almost 

 •certainly a more compact variety of the same rock, and that this 

 may have been a picrite ; also that the serpentine is probably the 

 intruder. But I can now throw a little light on the banded 

 structures, which are among the perplexities of the section. On 

 examining the dyke-like serpentine (3) I found that towards the 

 edges it gradually became banded. This structure on the right- 

 hand side began to be indicated about 2 inches from the exterior, 

 but only became very marked in the outer inch, where a grey tint 

 predominated. The red part, on microscopic examination, is found 

 to owe its colour to haematite-staining, and to consist of a 

 serpentine (mineral) thickly crowded with minute flakes of a clear 

 mineral, many of which give oblique extinction, and resemble actino- 

 lite, but others give straight extinction, being probably a chlorite. 

 The last inch, however, is practically colourless (only spotted with a 

 few grains of iron oxide), consisting of matted flakes, quite double 

 the length of those in the other part, but the same minerals. The 

 edge of the serpentine (5), where it is in contact with the small dyke, 

 exhibits a band about J inch wide. The red part resembles that 

 described above, except that it seems to be a rather purer serpentine, 

 .and it contains one or two ill-preserved grains of bastite, while the 

 band itself consists of a streak of fairly clear orange-coloured 

 serpentine, followed by the matted minerals just mentioned, in 

 which a kind of bandlet is formed by some small grains of brown 

 hornblende, with a little streaking of serpentine. I have also 

 examined a piece of the dyke, which is in contact with this face of 

 the serpentine. Probably it was once a basalt ; it has consisted 

 mainly of two minerals, each of which has been replaced by 

 secondary microliths : the clearer patches probably representing a 

 felspar, the others almost certainly a pyroxene. But attached to 

 this is a zone, which in the field I thought might represent a 

 junction. This bears a considerable resemblance to part of the 

 border in the other specimen, for it consists of a matted mass of 

 acicular minerals, some being considerably larger than the rest 

 and distinctly actinolitic, in which one or two grains suggest the 

 possibility of having been bastite. Besides these I have examined 

 -a piece of the striped red-and-grej 7 " rock (the supposed schist) in the 

 left-hand part of the crag, where the bands locally are sharply 

 folded. The red rock is a serpentine, like that already described ; 

 the grey one is a mass of microliths, generally too minute for iden- 



