170 3. A. PHILLIPS ON THE SO-CALLED 



In other specimens the crystals are exceedingly minute, and 

 closely matted together, without the stellate or banded arrangement 

 above described. All the sections examined contain magnetite, 

 which is often finely granular, and is seen, with a combination mag- 

 nifying 250 linear, to be in the form of imperfect crystals, with 

 rounded edges. Less frequently the magnetite is more distinctly 

 crystalline ; and when this is the case, it is generally, in part, re- 

 placed by some siliceous mineral. 



Occasionally the transparent granular base of which these rocks are, 

 to a large extent, composed, instead of containing radial or flocculent 

 crystals, encloses numerous irregular flakes of brown hornblende or 

 mica ; magnetite and viridite are also present in this variety. 



No altered gabbros or dolerites of the class found in the neigh- 

 bourhood of Penzance were observed in the Cape-Cornwall district ; 

 and the majority of the sections examined, nineteen in number, 

 afforded evidence of the slaty or schistose nature of these rocks *. 



Gurnard 's-He ad District. — The greenstones in the vicinity of the 

 Gurnard's Head, as laid down upon the Map of the Geological 

 Survey, extend, with certain interruptions, from that promontory 

 to Porthzennor Cove. An examination of these rocks in situ soon 

 renders it apparent that they very closely resemble those found in 

 the neighbourhood of Newlyn and Penzance. The slaty rocks, 

 which here, as elsewhere, dip from the granite, are perhaps more 

 crystalline and somewhat darker in colour than those of the former 

 locality ; the gabbros or dolerites, on the other hand, have under- 

 gone a still larger amount of metamorphism. 



The upper portions of the Gurnard's Head are composed of a 

 hard, dark, bluish-green slate, consisting of a transparent base, 

 through which brown and green microlites, with granular mag- 

 netite, are plentifully and regularly disseminated. Beneath this is 

 a dark crystalline rock, which, although apparently stratified, ex- 

 hibits no evidence of cleavage. Thin sections are seen to consist of 

 the usual transparent base, enclosing hornblende and viridite, with 

 pseudomorphs after magnetite, and small irregular crystalline 

 patches of augite or diallage. It also contains a few small and 

 much- altered crystals of felspar, together with various shadowy 

 patches and outlines suggestive of extensive metamorphism. 



A wide band of rock, coloured upon the Geological Map as green- 

 stone, immediately south of the headland, is shown by the micro- 

 scope to consist of a translucent granular base, containing grains of 

 quartz, throughout which the ordinary minerals are disseminated in 

 the usual way ; it also encloses a few minute garnets. 



A narrower band, a little south of the above, has sometimes a slaty 

 structure ; and sections of it are found to have a similar composition. 



The two next headlands north-east of the Gurnard's Head, form- 

 ing the extreme limits of Porthglaze Cove, are, to a great extent, 



* A greenstone from Botallack, analyzed by the author (Quart. Journ. Geol. 

 Soc. vol. xxxi. p. 329), was found to contain 48 per cent, of silica ; in all 

 other respects its ultimate composition closely agrees with that of the pyroxenic 

 rocks of Penzance, and the slaty blue elvans occurring between St. Erth and St. 

 Stephen's. These latter rocks do uot materially differ in chemical composition 

 from the ash beds of St. Kew. 



