ON THE CRYSTALLINE ROCKS OE TUK LIZARD DISTRICT. 481 



are, indeed, the most abundant rocks, but they differ much in 

 apj)earance from the normal liornblende-schists, in the minuteness 

 of their constituents and in the presence of a more acicular variety of 

 that mineral *. But the advance of our knowledge during the last 

 few years leads us to doubt the advisability of making any definite 

 separation. Three bands of brownish mica-schist, like that at 

 Polpeor, have been discovered by Mr. Fox (and visited under his 

 guidance by the authors), intercalated with the normal hornblende- 

 schist in Tolledan Cove, E. of Housel Bay t ; and the character 

 of the green schists may be accounted for by subsequent pressure 

 and shearing. The whole region has evidently been greatly modi- 

 fied since its constituent rocks first crystallized. The mica-schist at 

 Polpeor is crumpled, and the green schists often exhibit structures 

 resembling the " mylonite " of the Highland thrusts-fault region ; 

 tongues of a porphyritic diabase may be seen on the shore, so 

 crushed and sheared as to be barely separable from the green schists J. 

 On Old Lizard Head §, a cleavage foliation may be seen traversing 

 the corrugated banded greenish schist at a high angle, and the rock 

 below is in places a breccia of a gneissoid rock and of a rather soft 

 " green schist " in hopeless confusion, very suggestive of faulting. 

 Polkerris Cove (S. of Porthalla), in which some serpentine and a 

 little gabbro occur, affords evidence confirmatory of this view. On 

 the northern side we find a flinty-looking schistose rock (very 

 similar to one variety at Polpeor), and can identify in one place a 

 porphyritic dyke, rather like that named above. The flinty rock is 

 sometimes porphyritic, and may be a modified dyke, but other parts 

 suggest affinities with the hornblende-schist, into which there is a 

 passage on the southern side of the Cove. A specimen from the 

 northern side, which in the field seemed more nearly related to the 

 hornblende-schist than to the diabase, has been examined. It 

 shows marked indications of crushing and shearing ; fragmental 

 " eyes " of rotten felspar or of hornblende (sometimes very like 

 altered diallage) occur in a sort of mosaic of minute hornblende and 

 felspathic grains (possibly also of quartz), with a sort of " fluxion 

 structure." One part of the slide is coarser and still retains traces 

 of a fragmental structure ; another consists of thin bands of a 

 mosaic, in which this or that mineral predominates. Thus the 

 passage of the normal hornblende-schist into a rather flinty-looking 

 schistose rock in consequence of shear seems to be demonstrated |j. 



* See description of the principal varieties, Quart. Jom*n. Geol. Soc. vol. xxxix. 

 (1883) p. 12. 



t Described by Mr. Fox in Trans. Key. Geol. Soc. Cornw. vol. xi. pt. v. 

 (1891). A similar mica-schist occurs near Pistil Ogo, but here in the 'green 

 schist.' 



J A larger and less disturbed mass occurs a little farther east, and is 

 described in Quart. Journ. Geol. Soc. vol. xxxix. (1883) p. 4 



§ Name on the six-inch map ; called ' The Quadrant ' in Prof. Bonney's paper. 

 That name is now applied to an island below. 



I The clilfs and sliore do not afford a continuous section, so that a fault may 

 escape notice. As there is a fault at Porthalla and must be one at Porth- 

 oustock, this, the only intermediate cove, may be also determined by a fault. 



