34 PROF. T. G. BONNEY ON THE [Feb. 1 896,, 



minerals ; in others this ' matrix ' is the orange- coloured serpentine 

 already mentioned. Other streaks consist of this serpentine with 

 rather small pyroxenic grains. All these varieties are associated in a 

 slag-like fashion (PL I. fig. 6). A slice from another specimen, rather 

 more serpentinons in aspect, exhibits a junction of two kinds of reek- 

 One presents the general structure of a hornblende-schist, but the 

 grains of that mineral are brown, while the others, instead of being 

 felspar, are commonly a brownish-orange or greenish serpentinous- 

 mineral, which in some cases, when examined with crossed nicols,. 

 resembles a bastite, in others prove to be a granular aggregate. The 

 other kind of rock exhibits a very marked nuxional structure in a 

 matrix (if one may so call it) which seems to be composed of minute 

 doubly-refracting minerals, and its bands are darkest when parallel 

 with the vibration-planes of the crossed nicols. This is spotted with 

 a brown hornblende, which occurs both in granules and in grains of 

 the usual size, and the above-named structure is almost at right 

 angles to a very slight parallel ordering of the grains in the other 

 part (PI. I. figs. 3 and 4). 



This, however, is not all. Let us examine a piece of hornblende- 

 schist which was included in the porphyritic dyke. 1 It is a fairly 

 well-banded variety, thoroughly typical, except that it is a little 

 duller or of a more ' greasy ' aspect than is usual. 2 Under the 

 microscope we find a rather marked difference between the bands. 

 Those which are lighter coloured in the hand-specimen, and are 

 spotted with white in reflected light, consist of a very decomposed 

 felspathic mineral, and of pyroxenic minerals, some (the less nume- 

 rous) brown hornblende, others a pale green rather fibrous hornblende,, 

 and yet others consisting of aggregates of minute minerals. In 

 certain bands the brown hornblende dominates, and some felspar still 

 remains, while in yet another band the granular structure has 

 wholly disappeared, and the rock consists of numerous colourless 

 flakes or prisms (about '02 inch long), which have the extinction of 

 actinolite, and are set in a mass of smaller flakes, probably, in part 

 at least, a white chlorite. Here then a specimen of hornblende- 

 schist, once normal, which has been affected by ' contact-metamorph- 

 ism,' unites the characteristics to which I have just called attention 

 and those mentioned in our last paper, 3 as belonging to supposed 



1 A specimen of this dyke, taken from close to the included hornblende-schist, 

 has been examined with the microscope. In one part of the slide there are 

 indications of mechanical disturbance. This, however, is almost certainly due 

 to some accidental cause, such as a slight faulting, which escaped notice in the 

 field ; for the porphyritic felspars are uninjured, and the other parts exhibit a 

 hypophitic structure. The felspar is generally decomposed, and the augite has 

 been replaced by a dirty-looking secondary product, which shows a composite 

 structure with crossed nicols. The rock is readily distinguishable from the horn- 

 blende-schist. 



2 Its specific gravity is 2*917, that of a speckled variety from S. of Kilcobben 

 Cove 2-986, that of a banded (epidotic) variety from Church Cove 3074. The 

 specific gravity of the serpentine in this district is 2-730 (pit between Mullion 

 and Lower Predannack) and 2 - 766. The specific gravity of two of the specimens 

 of the streaky rock described above is 2-935 and 2-954. This is significant. 



3 Quart. Journ. Geol. Soc. vol. xlvii. (1891) pp. 472-73. 



