Notices of Memoirs—A Head of Hybodus. — 427 
duced a highly schistose structure. Its various mineral constituents 
are much crushed, flattened and broken up into fine fragments, 
the long axes of which conform more or less to the planes of 
cleavage produced by the great mechanical pressure the rock has 
here undergone. The original porphyritic structure, though much 
crushed, is “yet distinctly traceable, and further north, where the 
rock is less cleaved or schistose, as towards Porthkerris Cove, and 
Point, the porphyritic structure much abounds. 
There is another point with regard to the Porthonstock greenstone 
which must not be overlooked, that is, the tendency of the rock in 
parts to assume the granulitic structure, which, although on a small 
scale here, is yet the same type of rock as met’ with in the southern 
-areas. This also is, I contend, but a portion separated from the 
greenstone by segregation, so that from the parent mass of the gabbro 
has been evolved the. granulitic and the greenstone, which latter by 
subsequent dynamical movements has “been converted into horn- 
blende-schist. 
This explanation I am inclined to regard as no mere speculation, 
but as a fair and just inference based on what we can observe in 
many localities at the Lizard. In some of these localities all three 
rocks, gabbro, granulitic, and hornblende, are more or less inter- 
changeable, and a distinct passage can be traced between them all. 
In certain areas large tracts of the latter rock, which seems to have 
formed the upper or outer margin of the gabbro, have been cleaved 
into what now form the schists, while the more granulitic portions, 
although closely adjoining, have from their coarse and granular 
nature been much less affected. 
It will, I think, be ultimately found that segregation has played 
a most important part among all the rocks of the Lizard district. 
The bands of hornblende so frequently occurring in the serpentine 
in various localities are due, J think, to this cause. The dykes in 
the outer rocks off the Lizard Mende mapped with such care, and 
not without danger, by Mr. Howard Fox, F.G.S.,’ are, I believe, 
true segregation dykes. The banded structure in the hornblende- 
schists and associated gneissic rocks is also in my opinion due to 
this same cause, a subject I hope to deal with very shortly. 
If the present suggestion is correct, in reducing the triple division 
of the rocks at the Manacle Point to mere varieties of one original 
magma, it seems to go a long way towards simplifying the seology 
of the rest of this most interesting district. 
iO ae bC i S51 Oi Iver VE} ees 
1. “On a Heap or Arzopus DrtaBecHrI, ASSOCIATED WITH DorsaL 
HIN-SPINES, FROM THE Lower Lias or Lyme Recis, Dorser- 
suire.” By A. Smrra Woopwarp. Ann. Report Yorksh. — 
Soc. 1888, pp. 58-61, pl. i. 
HROUGH the generosity of Mr. William Reed, F.G.S., the Mode 
shire Philosophical Society is enabled to publish, in its recently- 
issued Report for 1888, a fine quarto plate (drawn by Miss G. M. 
1 On the Gneissic Rocks off the Lizard, Q.J.G.S. May, 1888, p. 309. 
