NEIGHBOURHOOD OF ST. MINVER, CORXWALL. 393 



•consisting of identical or unmixed lava fragments, in which case it 

 is hard to distinguish such a deposit from a similar coulee of lava 

 which has undergone crushing in situ. 



To Mr. Grenville Cole, whose knowledge of vitreous rocks is both 

 extensive and exact, I am also indebted for a confirmatory opinion *. 



The following is a short account of the nature of these rocks : — 



No. 1. Cant Bill, East end, at top of hill (Plate XII. fig. 1). — 

 This is a light brown, roughly fissile rock with greyish or yellowish- 

 white porphyritic crystals, chiefly less than an eighth of an inch in 

 diameter. On a surface cut transversely to the direction of fission, 

 the general structure of the rock is rendered apparent by exceed- 

 ingly delicate sinuous streaks of three kinds, namely dark green, 

 reddish-brown, and grey. These fine streaks are seen here and 

 there to be traversed by very delicate veins of a dark-green colour. 



Under the microscope the finely-streaked appearance of the section 

 is at first sight strongly suggestive of the fluxion structure seen in 

 lavas. There are bauds and small knots of felsitic matter (micro- 

 crystaUiue) separated by bands of translucent greenish-yellow or 

 brownish- yellow serpentine t, and more delicate opaque streaks, 

 some of which are reddish brown or rust-coloured, while others 

 appear snow-white. These opaque streaks seem to indicate, in part, 

 an original fluxion-structure in the rock, especially in the case of the 

 snow-white markings. In one part of this section there occurs a 

 thin vein of greenish-yellow to brownish-yellow serpentine, which 

 is traversed by the opaque rust-coloured strings, while the opaque 

 white streaks are distinctly separated by the vein ending abruptly 

 against its edges. This clearly shows that we have to deal with 

 two distinct sets of opaque streaks — the one set having been formed 

 before, and the other after the formation of the vein. The white 

 streaks represent the earlier and the rust-coloured streaks the later 

 structures. That the rust-coloured markings do not represent 

 fluxion-structure is evident, since, if they did, they would be 

 interrupted by the vein of serpentine ; but they traverse it. The 

 white markings, on the other hand, do not cross the serpentine vein, 

 and it is therefore possible that they indicate an original fluxion- 

 structure in the rock. Both sets of these opaque strings run 

 confusedly together, and interlace in a complicated network, except 

 where the vein occurs. 



The greenish-yellow serpentinous films and strings which follow 

 the general striping of the section are found, when examined in 

 polarized Hght, to be actually continuous with the similar substance 

 of the vein which traverses them, as shown at a a in the accompany- 

 ing woodcut (fig. 1), while the black markings indicate the opaque 



* Shortly before the reading of this paper Prof. Bonney also examined these 

 specimens and sections, and lindly gave me his opinions on them. In the main 

 his conclusions agreed with those already arrived at. One or two of the points 

 upon which he diffei'ed have since been recorded in supplementary notes. 



t Prof. Bonney, who favoured me with an opinion upon these sections, regards 

 much of the substance which I have here called serpentine as a palagonitic 

 material. 



2e2 



