314 MR. J. J. H. TEALL ON ROCK-SPECIMENS COLLECTED 



ledges to the north of it, are more basic than the other rocks, and 

 appear to dip ii.W. by W. 



" Labham Eock " is distinctly gneissic. " Labham E-eefs," which 

 can be reached at extreme low water without a boat, are apparently 

 transition rocks, and present in close association rocks in various 

 stages of transition from the brown micaceous schists of Polpeor to 

 the distinctly gneissic rocks. The "Labham Eeefs" appear to be 

 conformable with the Lizard-Head schists. 



All the foregoing rocks are covered at high water excepting 

 Mulvin, Man of War, Quadrant, Yv'iltshire, Shag rock, and Labham. 

 Some are uncovered only at extremely low spring-tides. 



Notes on Eock-specimens collected hy Mr. Fox from the Islands off 

 tlie Lizard Head. By J. J. H. Teall, Esq., M.A., F.G.S. 



The two small islands of Men Hyr and Yasiler lie half a mile due 

 south of Polpeor. A line drawn between these islands in a I^.W. 

 by W. direction passes through the Clidgas Eocks, the Man-of-War 

 Eocks, and the Stags. These islands, together with Taylor's Eocks 

 and Mulvin, which lie slightly to the south-west of the above line, con- 

 stitute what may be termed the Outer Group. Inside these we have 

 Enoch Eock, Pen Ervan, Labham Eeefs, Labham Eocks, Men Par, the 

 Shag Eock, Wiltshire, the Quadrant, and Sanspareil. We will now 

 describe the rocks from the different islands, taking the latter in the 

 order mentioned. 



Outer Groitp. 



Men Hyr. — A coarse gneissose rock showing a marked foliation. 

 Dark lenticular patches, rich in ferro-magnesian minerals, alter- 

 nate with light-coloured patches of similar form composed of quartz 

 and felspar or pseudomorphous sabstances after felspar, tinder 

 the microscope the rock is seen to be composed essentially of 

 felspar, quartz, dark mica, and hornblende. The felspar is so altered 

 in places as to have lost all individual action on polarized light •; 

 it is then represented either by aggregates of a vividly polarizing 

 scaly mineral (mica) or by patches which appear opaque by trans- 

 mitted, and snow-white by reflected light. The unaltered felspar 

 is abundant in certain portions of the slide ; the larger grains are 

 mostly striated. Quartz occurs in grains and granular aggregates. 

 The quartz and felspar in certain portions of the slide exhibit the 

 relations characteristic of igneous rocks ; in others they form a fine- 

 grained granulitic aggregate. 



Brown mica occurs in scales which are often arranged with their 

 flat surfaces lying roughly parallel to each other. 



Hornblende occurs in grains, often elongated in the direction of 

 the vertical axis, but without definite crystalline outline : — a, pale 

 brown ; /3, green ; y, bluish-green. Iron-ores and a few small 

 garnets occur as accessory constituents. 



The rock possesses the mineralogical composition of quartz-diorite 



