ASSOCIATED ROCKS OF PORTHALLA COVE. 463 
Detailed Description of the various Rocks. 
I will now proceed to describe in somewhat more detail the more 
important of the various rocks as they succeed each other. 
I have made analyses of nearly all the rocks referred to, and also 
prepared sections for microscopic examination. As regards their 
minute structure, I have little to add to the published descriptions of 
Prof. Bonney, except to remark that the serpentine differs extremely 
from that of Coverack and Cadgwith, and seems to me to be entirely 
destitute of any indication of the former existence of olivine. 
The Slate-—This is the rock which forms both sides of the Cove, 
and through which the rivulet makes its way. On the north side 
it consists mainly of dark brown to black shales or mudstones, 
sometimes traversed by thin veins of calcite and others of oxide of 
iron. On the south or, rather, east side of the Cove, these rocks form 
low cliffs, generally pretty hard from the presence of innumerable 
veins or kernels of quartz, and occasionally spangled with minute 
shining plates of mica, especially near the faults to be referred to 
hereafter. As already stated, the dip of these beds is to the 8.8.E., 
increasing from about 10° on the north side of the Cove to 50° or 
more on the east side near the fault. The strike differs but little 
from that of the ‘‘ Ladock beds,” which I consider to be Lower 
Devonian ; and it is quite possible that these beds are of the same 
age. At any rate I agree with Professor Bonney in thinking them 
newer than the rocks on the east side of the fault. 
The slates are traversed by a ferruginous fault or lode, which runs 
somewhat to the south of east, and dips to the west of south about 
60°, as indicated on the accompanying map (fig. 1). This lode comes 
out on the shore in a little cove about 180 yards to the eastward 
of the Porthalla rivulet. It contains some concretionary masses of 
siliceous brown hematite, and has yielded a few minute crystals and 
grains of ilmenite *. In some places this lode-fissure has followed a 
“line of least resistance” between the “ granulite” and the rock 
which it traverses. The lode may be followed along the shore near 
high-water mark for about 20 or 30 paces, the slate in its vicinity 
being very highly charged with siliceous matter, and very often 
exhibiting a brecciated appearance. It is then traversed “and 
heaved” by a fault running about N.E., and dipping very steeply to 
the south ofeast. At this point the lode contains very little oxide of 
iron; it is about 22 feet wide, as measured upon the beach, or per- 
haps a little more. The slates here are much contorted, that is, in 
the immediate neighbourhood of the fault. 
Another fault, apparently parallel to that just referred to, and 
thus pretty nearly running in the strike of the rocks, exists a little 
further on. This may perhaps be assumed as the principal cause of 
the shattered condition of the rocks in the little cove immediately 
succeeding the ‘‘ heave” above mentioned. It is no doubt “the 
*' The first and almost the only crystals of ilmenite which I have ever met 
with in Cornwall were sent me by Mr. Howard Fox, some twelve years ago, and 
came from this very lode at the point referred to above. 
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