108 W.M. Hutchings—Altered Igneous Rocks, Tintagel. 
of the sheet, but it was not possible to make sure whether this was 
the case, or to get any idea of its extent. 
The microscope shows this rock to be made up of much flea 
with a great deal of chlorite, some secondary quartz, a little calcite, 
a few flakes of biotite, and large amounts of epidote and of granular 
sphene. 
Most of the felspar is water-clear and indefinite, but there are 
also a good many well-defined columnar forms, of various sizes, 
showing twinning, binary, and multiple. In the amount of in- 
dividualized felspar this rock stands out quite distinct from any 
other specimens from the sheet, as it does also in regard to quantity 
of felspar present. 
Many of these felspar crystals are bent and broken and show 
curved twinning-lines, and a very large proportion of them, as well 
as of the water-clear bits and the quartz grains show strongly undu- 
lous extinctions. The epidote also is much broken up and the ends 
of many crystals are surrounded by quantities of small crushed 
fragments. But for the absence of foliation, this rock seems to have 
undergone quite as much stress as any other part of the sheet. 
Were it not so, one would be rather inclined to think it might be a 
later igneous matter intruded into the already altered rocks of the 
sheet. It appears, however, more likely, in view of the similarity 
of its alteration-products, both in nature and extent, to those of the 
other specimens examined from various parts of the sheet, that it is 
really a part of the same original mass, which has escaped foliation 
for some reason we cannot explain. Were this foliated like the rest, 
it would differ only from some other specimens in being more fel- 
spathic; and even this difference might very probably disappear, 
owing to development of mica and other minerals from the felspar, 
which there is reason to believe would be brought about during the 
process of foliation. 
A thorough study of this interesting sheet of rock would require 
further attention in the field, and the examination of a much larger 
series of sections than I have prepared. From what I have adduced 
above it seems reasonably certain that the entire mass is of igneous 
origin, but there does not seem to be any basis whatever on which 
to decide as to whether the original material was a massive rock, 
consolidated from a molten condition, or whether it was a fragmental 
deposit of tuff or ash. It seems likely that where such great altera- 
tions have taken place as are here shown, all clue to the original 
nature and condition of the rocks would be equally destroyed in 
either case. 
Whatever was the original nature of the material, we see from the 
microscopic record that even prior to the great earth-movements of 
the district it had undergone very complete alteration under the 
ordinary influence of chemical action, and doubtless much pressure 
from the masses of sedimentary rocks piled above it. Secondary 
hornblende had formed and probably mostly passed in its turn into 
chlorite. Calcite was everywhere abundant and much quartz had 
been deposited. 
