470 PROF. T. G. BONNF.Y AND MAJOR-GEN. C. A. M'^MAHOX 



extent a fluidal structure — may be detected. The rock, however, 

 is usually rather rotten ; very commonly there is a dusty-looking, 

 pale-coloured, more or less chrysotilic layer between the two rocks, 

 which is continuous with the serpentine, but does not adhere to the 

 granulitic mass. Indeed we have never found the two rocks 

 actually welded. Signs of crushing and slickensides may no doubt 

 be not unfrequently seen at junctions. This is only to be expected, 

 because the tenacit}' of the two rocks is so different that, even if 

 welded, they would have parted here under strains from ordinary 

 earth-movements. The serpentine also at the margins is often 

 decomposed, and its structure is obscured by secondary chrysotile, 

 steatite, and other like minerals. The serpentine, which seems to 

 include these gneissoid masses, sometimes exhibits a slight streaki- 

 ness, the significance of which will be considered presently. Some- 

 times this may be parallel to the apparent bedding of the granulitic 

 rock, but at others the two structures are almost at right angles, so 

 that evidently they are not necessarily connected. 



The relations of the serpentine to the hornblende-schist differ 

 somewhat from those with the Granulitic Group. Masses of the 

 latter, apparently included in or pierced by the serpentine, may 

 be found by scores ; with only one or two slight intervals, they 

 literally fringe the eastern coast for a distance of three miles, mea- 

 sured along its curve, and they are not unknown, though much less 

 common, on the western coast *. But the Hornblendic Group is not 

 often seen actually cut by or entangled with the serpentine. This, 

 however, may also be said in regard to the other intrusions. Dykes of 

 any kind, so far as we know, are not very common in it. Still, though 

 the serpentine and hornblende-schist in some cases may be faulted 

 together, the relations in a few are clear. At Henscath, just north 

 of Mullion Cove, we find it impossible to explain by earth-movements, 

 or by any theory but that of intrusion, the position of the appa- 

 rently insulated mass of serpentine on the little headland, and the 

 two rocks, in at least one case, are still welded together. Again, 

 the same holds good in regard to the junction exposed in the upper 

 rocks near Pare Bean Cove (north of Ogo-dour Cove f)- Strips of 

 the hornblende-schist, regularly banded, are split off by or included 

 in the serpentine, and the two rocks in more than one place are 

 welded together, though the rottenness of the latter makes it im- 

 possible to detach and bring away specimens exhibiting the 

 junction. Moreover, the hornblende-schist, thus included, exhibits 



* The masses at Xynance Cove, mentioned in my paper, Quart. Journ. 

 Geol. Soe. vol. xxxiii. (1877) pp. 884-928, certainly in some cases, and 

 perhaps in all, are better referred to the Granulitic than to the Hornblendic 

 Group. The road descending to the cove (p. 888) crosses a mass wliich, 

 vrhen less clearly exposed, was mistaken for a granite vein. There are other 

 characteristic masses along the shore N. of Pentreath beach ; some of these are 

 mentioned as hornblende-schist in the above paper, in which no division of the 

 schists is attempted ; they were not again examined for the purpose of writing 

 the second paper, the precise reference of every block being unimportant. — 

 T. G. B. 



t Bonney, Quart. Journ. Geoh Soc. vol. ixxix. (1883) p. 22 (referred to as 

 being ' to the north of Ugethawr '). 



