494 PROF. T. G. BONNET AND MAJOR-GEN. C. A. M'^MAHON 



appears separable from the above-named groups of dykes. It seems 

 often to break up the gabbro, and then to cement the fragments, so 

 that the two form one mass, while the above-mentioned cut clean, 

 as dykes, through both, being sometimes welded, sometimes separable. 

 It is slightly speckled, somewhat dark on freshly fractured surfaces, 

 weathering a rather warm grey, sometimes porphyritic (felspar), 

 sometimes green-spotted. A specimen from near Manacle Point 

 consists of plagioclase, augite, partly altered into a brown horn- 

 blende, altered olivine, and granular magnetite; it is therefore a 

 fine-grained gabbro. The boundaries of the grains are very irregular, 

 and the augite not seldom includes either lobes or grains of the fel- 

 spar *. The normal gabbro presents a similar structure, but has less 

 magnetite, and the pyroxenic constituent is either diallage or is 

 altered to a fibrous actinolite, with a little of the brown hornblende ; 

 the felspar also is more decomposed. The boundary between the two 

 is not very sharply defined under the microscope. Macroscopically 

 the dark rock at Pen Yoose, which is similarly associated with the 

 gabbro, much resembles the above, but in the three specimens ex- 

 amined hornblende (green) alone is present ; magnetite is scarce in 

 this rock. This also has a granular structure, but the individual 

 grains are smaller and rather more regular in shape, so it differs 

 more conspicuously from the adjacent gabbro (in which also the 

 augitic constituent is replaced by hornblende). This rock was pro- 

 bably an early intruder ; nevertheless, at that time the gabbro was 

 not only crystalline but foliated, as can be seen on careful scrutiny 

 in one or two instances at Pen Yoose, for the subangular fragments 

 of foliated gabbro are scattered in the dark matrix as if they were 

 bits of a schist f. 



Lastly, there are two dykes which differ in some respects from all 

 those already mentioned. The others have a distinctly rhyolitic 

 aspect. One occurs on the road leading from Landewednack fco the 

 back, or sea-face, of the serpentine quarry between Church Cove and 

 Pen Voose, forming a vein a few inches thick in serpentine. As it 

 vreathers to a similar colour, the outcrop is easily overlooked J. 



* Compare plate iv. figs. 2, 3, illustrating Prof. Judd's paper on * Tertiary Gab- 

 bros,' &c. in Scotland and Ireland in Quart. Journ. Geol. Soc. vol. xlii. (1886). 



t I made a mistake in regard to the relations of these two rocks at the time 

 of my earlier visits, which affects a few lines in my first paper, viz. those on 

 p. 894 : — ' The gabbro and hornblende-schist are here mixed up in- 

 trusive.* The close resemblance, macroscopic and microscopic, of the rock 

 described above to some of the less foliated and unhanded varieties of the horn- 

 blende-schist (or the darker part of the Granulitic Group) led me to suppose 

 that the gabbro was the intruder, and had acquired its foliation from pressure 

 in cooling ; but, on re-examination, I find that there are difficulties which did 

 not then occur to me (for parts of the supposed hornblende-schist closely 

 resemble an unmodified igneous rock), and that the foliation in the gabbro can- 

 not be thus explained. The evidence, even at Pen Yoose, now appears to me 

 more favourable to my present view ; and that which we obtained in the neigh- 

 bourhood of Manacle Point, where there are similar appearances in a less altered 

 rock, seems convincing. This, however, does not affect the general argument of 

 that part of the paper. — T. G. B. 



I It has been examined by Gen. M'Mahon. 



i 



