322 =T. G. BONNEY AND F. T. 8S. HOUGHTON ON THE METAMORPHIC 
present as well as orthoclase. The structure of the rock, as ex- 
plained below, suggests that it belongs to the granitoid gneiss group 
rather than to the true granite *. 
The next exposure of rock which we visited set our doubts at 
rest. It is in a pasture-field, where, on the 8.E. side of the ridge, 
and perhaps a furlong to the N.E. of Twt Hill, is an old quarry. 
This affords the section shown in the sketch (p. 321). The lowest 
rock exposed is of the same type as that of Twt Hill. Above it comes 
a bed with a similar matrix, containing pebbles in the upper part; 
then a band of coarser conglomerate, the pebbles being often full an 
inch in length. To this succeeds a band of finer conglomerates, 
then of coarser, and, lastly, another of finer, passing up into a rock 
which has some resemblance to the bottom rock. ‘The pebbles are 
almost all vein-quartz, and are rather flattened; one or two look 
like an impure jasper. The strike of the conglomerate is about 
E.N.E., and the dip about 60° on the southern side. In the finer 
bands the pebbles appeared more rolled than in the coarser, varying 
from the size of hemp-seeds to that of peas. The matrix of the 
most conglomeratic part is a kind of quartzose grit. Microscopic 
examination, as will be presently shown, fully confirms the above 
evidence as to the clastic origin of this rock. 
On the N.W. side of Twt Hill are quarries showing a rock more 
distinctly gneissose than it is at the summit, the component minerals 
haying often rather a banded arrangment. One block, observed by 
Mr. Houghton, showed very well an alternate banding of coarser 
and finer varieties of the rock. The rock also in the higher part of 
the quarry became finer in texture y, and contained considerable 
quantities of a decomposed green mineral like epidotet; some 
quarried blocks in a field showed bands of grey grit interbedded with 
the normal Twt-Hill type of rock. 
On the right-hand side of the new road leading to Llysmeirion is 
a rock of the same general character, but paler in colour and more 
metamorphic in aspect. It seems to lie a little below the conglo- 
merate, and to overlie those exposed in the last pit, which are part 
of the general mass of Twt Hill. The rock from the pit near 
Llysmeirion has already been noticed §. 
Specimens from some of the above beds have been examined mi- 
croscopically. Commencing with a section || cut from the finer part 
of one of the conglomerates (which is more granitoid in aspect than 
some of the other bands), we find the rock obviously (as we should 
* A name appears to be needed for these granitoid rocks, which in general 
aspect much resemble a granite poor in mica, are metamorphic clastic rocks, 
but differ from ordinary gneiss in being scarcely, if at all, foliated, as well as 
in ae ae amount of mica. I venture to propose the name “‘ Granitoidite.” 
—T. G. B. 
+ A fine gritty-looking rock may also be seen on some waste ground higher 
up the hill, near a white cottage. 
¢ Specimens from these quarries exhibit considerable variety in the relative 
amounts of quartz, of felspar, and of the green constituent, some of which is more 
like a kind of chlorite. 
§ Quart. Journ. Geol. Soc. vol. xxxiy. p. 145. 
|| Kindly lent to me by J. H. Marr, Esq. —T. G. B. 
