554 T, DAVIES ON THE MICROSCOPIC STRUCTURE 
Seat. Consists largely of small quartz pebbles, some of which are 
clear, others of the dirty Dimetian type. Encloses a small but 
distinct pebble of quartz and felspar. 
No. 51 is a conglomerate from near Chanter’s Seat, the pebbles 
of which are mostly small. Among numerous pebbles of more or 
less dirty quartz are several of plagioclase and microcline felspar, 
much altered, and several small pebbles of the dirty quartz with 
plagioclase. A small pebble of a much altered basic rock is present 
also. These pebbles are not closely aggregated, but are cemented 
together by a dark-grey cement. 
No. 52. Cambrian from near Chanter’s Seat. Contains quartz 
in small fragments with felspars much altered. The cementing 
material is here very dark and opaque, probably from the presence 
of much iron-oxide. Encloses a pebble with associated quartz and 
decomposed felspar. 
No. 53. Grit from Chanter’s Seat. Macroscopically an arkose- 
like rock consisting of rounded quartz and decomposed felspars. 
Microscopically it is found to consist of the same minerals, the 
plagioclase and orthoclase being more rounded than the quartz 
and much altered, in some instances almost entirely replaced by 
quartz and the greyish-green mineral so frequent in all the rocks of 
this area. Some small pebbles are present, the structure of which 
resembles that of a basic rock. 
No. 54. Cambrian from near Chanter’s Seat. This is similar 
to 52. Contains a pebble of dirty quartz and altered plagioclase, 
with some decomposed felspar. 
Nos. 55-60. Cambrian grits &c. from Chanter’s Seat and beds 
below. Prepared sections of these conglomerates or grits enclose 
small but well-defined pebbles of dirty quartz associated with altered 
felspars. 
By far the larger portion of the quartz pebbles have also the 
characteristic cloudiness of that of the Dimetian. The clearer 
pebbles are suggestive of having been derived from quartz-felsites. 
Numerous rounded crystals and grains of altered felspar, some of 
which can be distinguished as plagioclase, are shown in all the 
sections, though varying in amount. Some of the sections enclose 
fragments which strongly resemble the more quartzose parts of the 
fine-grained Dimetian. Fragments of quartz-schists with mica, and 
traces of individual mica plates are present, but very sparsely. 
No. 61. Cambrian conglomerate from Caerbwdy Valley. Con- 
tains some quartz, like the preceding, fragments of a felsite much 
altered, and also a fragment of a porcellanite. More of the fel- 
spathic cement is present in this section. 
No. 62. Cambrian grit in contact with Dimetian, from Porth-clais. 
Consists almost entirely of dirty quartz, but with more of the 
felspathic cement, which also carries smaller quartz grains. There 
are some rounded fragments of a felsite. . 
No. 63. Cambrian sandstone in contact with Dimetian, from 
Porth-clais. ‘his is a very fine siliceous grit, but it contains very 
