504 Alex. Somervail—Dykes in the Lizard Serpentine. 
of the dyke along with that of the serpentine on either side of it 
being the cause of the gully. 
From the main trunk of the dyke near its base a number of 
branches or veins are sent off which traverse the serpentine more 
in a lateral and diagonal direction than a vertical one, as does the 
parent mass. 
These processes when taken together display a wonderful variety 
of rocks, most of which are to be found in the main dyke. These 
rocks in their extremes vary from a dull-looking trap, which many 
would term a dolerite, to a dark lustrous diorite, full of sparkling 
hornblende, then a greyish and also a reddish granite, followed by 
milk-white quartz, also masses of flesh-coloured felspar, with large 
plates of biotite, as well as some intermediate varieties of these 
rocks. Ata height of about 60 feet from the beach, in the main 
dyke on the north side of the gully, the diorite, granite, etc., are 
well mingled together, and other alternations are found at other 
portions, as well as in the branches where the dull-looking trap 
occurs. ° 
The inference to be drawn from these facts is, I think, that all the 
minerals and rocks mentioned have been differentiated out of the 
same magma during the cooling process, the ordinary selective law 
of chemical affinity separating the basic from the acidic types. 
The lesson to be derived from this dyke also seems to me to have 
a very important bearing on the other rocks of the same area. 
De la Beche, followed by Prof. Bonney, regards the schists, 
serpentine, gabbro and the granitic veins as belonging to widely 
separated intervals of geological time. When these are viewed in 
their massive relations to each other, there seems much to support 
this view, an opinion in which I also at one time partially shared ; 
but repeated examinations have shown me that certain relations 
which held good in one district were very obscure, or flatly contra- 
dicted, in another. In fine, the whole of these rocks (schists 
included) really seem more or less to interlace each other, and to be 
the product of one great period of eruption, if not the product of 
the same magma, heterogeneous or complex it may have been, 
rendered still more so, by cooling under different conditions. 
Among other proofs of this, there is what seems to me a very 
decided transition of the various Lizard rocks into each other, not 
promiscuously perhaps, but by way of the hornblende into the 
serpentine, also the hornblende into the gabbro. In the former case, 
as in the intermediate varieties of these rocks forming the islands 
and coast of Kynance Cove, and of many other localities. In the 
latter case, as in the curious mingling of the gabbro with the 
‘“‘granulitic”’ portion of the hornblende so often forming a gabbro- 
hornblende-schist, as at the Balk and Lean Water. 
The lesson of the dyke would yet further seem to teach us that 
however much subsequent dynamic action may have affected these 
Lizard rocks, their strongly contrasted varieties are much less due 
to this cause, even when combined with any chemical change which 
might arise therefrom, than to a highly heterogeneous magma, inter- 
