NEIGHBOTJKHOOD OF ST. MINVER, CORNWALL. 395 



in part) and its secondary products kaolin and felsitic matter, the 

 secondary product serpentine, which has probably resulted from the 

 decomposition of pyroxene or olivine, and the secondary product 

 limonite, which may have been derived from the alteration of magne- 

 tite or ilmenite. The normal constituents of the rock may there- 

 fore be regarded as those of a basalt or an andesite, while the greater 

 part of the felsitic matter present most likely represents the altera- 

 tion of a glassy base. We therefore seem justified in regarding this 

 rock as a once vitreous lava of a more or less basic type. To some 

 .extent I feel confirmed in this view by remarks made by Professor 

 Renard, who kindly examined these sections and gave me the benefit 

 of his "impressions" concerning them; and, although I had 

 previously arrived at similar conclusions regarding the mineral 

 constitution of these rocks, I must admit that I was inclined to 

 regard them as tuffs composed of fragments of once vitreous basic 

 lavas, rather than true coherent lava-flows. As it is, I cannot help 

 thinking that they have in many cases experienced considerable 

 crushing ; but, if so, it has been from pressure normal to the planes 

 of foliation, and the foliation is due, first to decomposition of certain 

 mineral constituents, and secondly to pressure ; but I can find no 

 evidence that any shearing action has been implicated in the 

 production of the structure which now exists in the rock. The 

 evidence afforded by the small serpentine veins transverse to the 

 direction of foliation negatives such a supposition. 



No. 2. Cant Hill, East end, at top of hill (Plate XII. fig. 3).— 

 This specimen was collected from the same locality as no. 1, in a 

 small grubbing not more than four or five feet deep. It is a pale 

 greenish- brown or greenish-grey rock, with numerous pale grey or 

 yellowish- white porphyritic crystals or flecks ranging from about a 

 fifth of an inch to quite microscopical dimensions. 



On a smoothly cut surface very delicate, wavy, but roughly 

 parallel streaks of a dark green colour are visible. A vein of a 

 dark green serpentinous substance cuts obliquely across the banding 

 in this specimen. The green matter has a hardness of about 3 or 

 slightly less. The rock weathers with a rusty brown surface. 



Under the microscope it presents the appeaj'ance of a vesicular 

 lava with a more or less devitrified ground-mass, and contains 

 numerous porphyritic triclinic felspar crystals, which have frequently 

 & fragmentary aspect. The vesicles are extremely numerous, in 

 most cases their sections are circles or ellipses, although they are 

 sometimes of irregular form, and they are for the most part filled 

 with serpentine, chalcedony, or quartz. The section is impregnated 

 with much opaque, yellowish-white matter, apparenlly kaohn. A 

 drawing made from this preparation, as seen by ordinary transmitted 

 light under an amplification of 2d diameters, is shown in fig. 3, 

 Plate XII. There is, in many parts of this section, an appearance of 

 contorted fluxion-structure. The extinction-angles of some of the 

 porphyritic felspar crystals indicate labradorite, others anorthite. 

 Some of these seem to have undergone a certain amount of corrosion, 

 iis suggested by Prof. Renard. 



