426 A. Somervail— The Lizard Greenstone. 
From the commencement of the greenstone on the south side 
of Porthonstock Cove, as indicated on the map, there is little more 
than a repetition of alternate greenstone and gabbro for a considerable 
distance to the south. At first the greenstone is the predominating 
rock for a short distance, then both almost in equal proportions. 
The gabbro in its turn, however, soon preponderates, yet still having 
a number of bands or veins of the greenstone, these becoming fewer — 
as the central mass of the gabbro is reached. 
These bands and veins of greenstone differ considerably from each 
other seemingly in proportion to their width and in their relations 
from the more central mass of the gabbro. Some of them are like 
dolerites, others like diorites, and both are frequently porphyritic, 
and not to be distinguished from similar rocks much further south 
associated with the granulitic group which cuts the serpentine. 
These veins, where I first saw them near the southern limit of the 
greenstone on the map, firmly impressed me with the conviction 
that they were due to segregation. During a moderate falling rain 
I was able to examine them to much advantage, the wet on the rock 
brings out the very decided way in which the veins shaded into the 
matrix of the gabbro. Repeated examples bore out the same con- 
clusion, and the broader bands pass in a similar way into the mass 
of the gabbro, without the slightest sign of intrusion, or of any force 
having been exerted, or the margins of either having been altered 
in the very least degree by contact. 
The example already referred to of the Porthonstock gabbro 
traversing the greenstone, so apparently contradictory, is also, I 
believe, due to segregation from the more extensive mass of the 
greenstone occurring at this point; so that on the ground of segre- 
gation, anomalies and difficulties at once disappear. I have no 
doubt that it was from the successive bands of both rocks at Por- 
thonstock Point, that De La Beche was induced to colour so much 
of this area as greenstone, if not from the very reason just given. 
There can be no doubt but that the greenstone and gabbro are por- 
tions of one and the same magma, the gabbro on its northern margin 
passing into the former, and as we shall presently see, the horn- 
blende schists immediately north of the greenstone are formed out 
of this latter rock,’ so that all three—gabbro, greenstone, and horn- 
blende-schist—are but the modifications chemically and mechanically 
of one mass, although represented by the three colours alluded to. 
From south to north, as already stated, there is a gradually growing 
disposition on the part of the gabbro to become more split up by 
the segregated greenstone veins and bands, until these become the 
dominant rock at and north of Porthonstock, resulting in what is 
now represented by the hornblende-schists north of that locality. 
The greenstone of Porthonstock is not by any means a rock that 
has undergone any very great amount of secondary alteration. 
Immediately north of it, however, there is evidence of a plane of 
great disturbance which has much more completely altered its 
original constituents, especially its augite into hornblende, and pro- 
' This opinion is expressed in Prof. Sedgwick’s paper, Trans. Cambridge Phil. 
Soc. vol. i. p. 18. } 
