Vol. 52.] 



ROCKS OE THE LIZARD DISTRICT. 



35 



cases of contact-metamorphism produced in hornblende-schist by- 

 serpentine. 



But before quitting this subject I must again refer to the locality 

 near Pare Bean Cove, north of Ogo-dour, 1 where I have already 

 asserted the serpentine to exhibit true intrusive junctions. Again, 

 perhaps for the fifth time, I studied this rocky slope with scrupulous 

 care. Here the serpentine has forced itself in a rather irregular 

 fashion through hornblende-schist, though it conforms on the 

 whole to the banded structure in the latter rock, slabs of which 

 appear occasionally to be broken off by and caught up in the 

 serpentine. 2 In this part the serpentine, which is sometimes 

 streaked, and in which the bands occasionally present some resem- 

 blance to hornblende-schist, is very rotten, but at a short distance 

 the rock is in better condition. A few yards to the west of this 

 complicated area, 3 a gully exhibits the junction of the principal 

 masses of serpentine and of hornblende-schist. Here, as shown in 

 the annexed figure (10), a slab of hornblende-schist is separated 

 from the main mass by a 



Fig. 10. — Gully north of Ogo-dour. 

 Intercalation of serpentine and 



' sill ' of serpentine. This slab, 

 where it was drawn, was about 

 3| inches thick, and it tapers 

 away in a yard or so to less 

 than 1| inch. That the horn- 

 blende-schist about here is 

 the ordinary rock cannot be 

 doubted; it exhibits both the 

 4 dioritic ' and the ' banded ' 

 varieties. The serpentine, 

 generally more or less decom- 

 posed, is a little ' platy ' in 

 the direction of and parallel 

 with the junction-surfaces, but 

 this structure is in no way 

 suggestive of crushing. The 

 serpentine also is occasionally 

 harder than is usual and looks 

 impure. It is locally streaked, 

 alternately warm brown and 

 brown-black in colour. When 

 the included hornblende-schist 



hornblende-schist. 



1 = Hornblende-schist (more or less 



banded, as at x). 



2 = Bottom of gully. 



Serpentine dotted. 



becomes very thin — say not more 

 than an inch in thickness, — it is sometimes difficult to fix upon the 

 exact line of junction, but where it is thicker, this is generally 

 sharply defined, and the weld between the two rocks is perfect. 



I have examined a specimen of this streaky serpentine (obtained 

 from near a weld) under the microscope (PI. I. fig. 1). The darker 



1 Quart. Journ. Geol. Soc. vol. xxxix. (1883) p. 22, and vol. xlvii. (1891) 

 p. 470 (with General M c Mahon). 



2 The bands occasionally are cut across by the serpentine. 



3 It could only be accurately mapped on a very large scale-map, and the 

 trouble of doing it would not be repaid by the result. 



D2 



