112 Prof. T. G. Bonney—Picrite in Sark. 
well as the microscopic study, show that the principal minerals 
occur approximately as follows: Hornblende, 58:5; altered olivine, 
22-0; altered mica, 18-5 per cent. JI think that the proportions in 
the Sark rock are not very different. Hence it is the result of the 
alteration of a picrite (using the word in the sense in which I have 
been accustomed to employ it), rather than of a normal peridotite, 
viz. of a group transitional between the latter and the dolerites.’ 
So far as we could see this rock does not occur in siti in the cliffs 
at Port du Moulin, but during both our visits there was a rising tide, 
and we were not able to reach nearly to low-water mark. Moreover, 
we found so much of interest in the hornblende-schist and its asso- 
ciates, that we had no time to search thoroughly the craggy slopes - 
above. I have, however, little doubt that the rock occurs in siti at 
no great distance. The boulder—more than half a cubic yard in 
solid contents—is far too large to have been brought as ballast,? for — 
it could only have been shipped by means of a crane, and nothing 
larger than a fishing boat (except in case of a wreck) would be 
likely to touch at Port du Moulin. Besides, I do not know of any 
seaport at which such a rock occurs. 
Since our visit, Mr. Hill has called my attention to the following 
passage in Ansted’s Geology of the Channel Islands (p. 264): “An 
important vein of serpentine and steatite, with asbestos and tale, 
has been traced crossing the central part (of Sark) near Port du 
Moulin.” This vein, Mr. Hill tells me, he did not find when 
engaged in the work for his paper on Sark; and as the term ‘ ser- 
pentine’ was often used rather vaguely at the time when Ansted 
wrote, he did not attach much importance to it, and the matter had 
even escaped his memory at the time of our visit. The coast of 
Sark is very precipitous, and if the dyke is not well exposed, it might 
easily be missed, even by so careful a worker as my friend. It 
would now seem probable that Ansted was substantially right in 
his identification, and that the rock which I have described comes 
from his ‘ vein.’ 
As there is no probability of my returning to Sark for years, if 
ever, I publish this note in the hope that some one will trace this 
interesting rock to its home. No doubt it will occur as a dyke, and 
there is a special interest in finding it here, because the hornblende- 
schist of Sark is lithologically identical with much of that from the 
Lizard peninsula, where varieties of serpentine are also intrusive in 
the crystalline series. 
I may add that, after my return to England, Mr. Hill found a 
picrite in Alderney, though it is nearer to the olivine-dolerites than 
that which forms the subject of the present note; this wil! be 
described in a paper which he is preparing on the geology of that 
island. 
Tt is, I think, inconvenient to apply the term picrite both to these rocks and to 
members of the peridotite group, composed of olivine and augite, where the former 
mineral dominates. and the percentage of magnesia nearly equals that of silica ; 
for the latter (probably rarer) form a new name might be coined, if augite-peridotite 
will not suftice. 
* Afterwards I noticed two or three others of smaller size. 
