78 



T. G. BONNET ON THE SERPENTINE AND 



far as I have seen, only occur on the shore in this one locality to the 

 north of Lendalfoot. Rising conspicuously above the serpentine 

 and the sand are two dykes of a hard whitish rock spotted with 

 brown. A short examination shows this to be a gabbro of plagio- 

 clase and diallage, in which, as at the Lizard, the former mineral has 

 been converted into a kind of saussurite. It always forms the 

 major part, and sometimes almost the whole of the rock. As it 

 resists the weather better than the diallage, the rock has often a 

 curious "pock-marked" aspect. Close at hand is a most remark- 

 able rock, best seen near the southern dyke of gabbro, though a 

 little of it also occurs near the other. This rock consists almost 

 wholly of coarsely crystallized submetallic diallage, the crystals 

 being often two or three inches long*. It breaks into the serpen- 

 tine in irregular branching veins, which die away in mere threads 

 (remaining rather coarse to the last like the gabbro at the Lizard) ; 

 in places it has almost shattered the serpentine, and includes frag- 

 ments of it. Its weathered surface, as might be expected, is extremely 

 rough, the diallage crystals projecting. The thicker parts of the mass 

 form little skerries. Here the rock may be 4 or 5 feet thick, but 

 generally it is in veins not more than a foot across. Careful exami- 

 nation left not the slightest doubt on my mind that this is a true 

 intrusive rock, and has not been formed by segregation or other- 

 wise. It seems at first sight connected with the adjacent gabbro 

 dj r kes ; but their obvious difference in mineral composition shows 

 they cannot be contemporaneous. The junction is also sharp, show- 

 ing no passage from the one to the other. A careful search settled 

 the question of their relative ages. The annexed diagram (fig. 4) 



Fig. 4. — Gahbros and terpentine. 



IHH Gabbro. j*J£^| Diallage rocks. j|] Serpentine. 



* Noticed by Dr. Geikie {he. cit. p. 527) as "vein-like ramifications of 

 diallage rock." 



