300 
POPULAR SCIENCE REVIEW. 
These had been partially described previously by Sedgwick and 
Salter, as Lower, Middle, and Upper Lingula flags. The 
fourth, or highest, group, was named by Sedgwiok, many 
years ago, ‘Tremadoc,’ and this name is still retained for 
that group. 
Maentwrog group (Lower Lingula flags).— The lowest beds 
in this group are described by Mr. Belt as ‘ grey and yellowish 
grey, fine-grained pyritic flags, with hard felspathic bands 
and rusty partings/ Alternations of more arenaceous and 
gritty beds are not unfrequent, and beds of yellow-grey 
shale also occur ; these are followed by bluish-grey slates 
and flags. The middle and upper beds also are chiefly 
flags and slates. At St. David’s, the lower beds are thin, 
ribbony-looking, and much contorted ; these are succeeded 
by dark iron- stained slates and bluish flags. The genus Olenus 
is the characteristic fossil in this group, and it and the 
Phyllopod crustacean Hymenocaris occur here for the first 
time. The group is divided into upper and lower series, as 
under : — 
o f Upper, j Slates and Flags, with. Olenus truncatus and Olenus cata- 
« p; I 1800 ft. ( ractes, &c. 
^ p / 
h o < 
p3 \ 
SO I Lower, j Sandy and Slaty Beds, with Olenus gibbosus , Agnostus 
£3 V 700 ft. \ pisiformis , &c. 
Ffestiniog group (Middle Lingula flags). — This is also divided 
into upper and lower portions. The lower consists of a series 
of slightly arenaceous and micaceous grey flags ; these are 
succeeded by thick beds of yellowish- grey arenaceous flags 
containing hard felspathic layers. The beds in this group are 
usually easily recognizable, because of the thin alternations of 
hard and soft layers, and by their peculiar weathering. 
Lingulella Davisii and Hymenocaris vermicauda are the 
chief fossils in the lower beds. The upper beds are chiefly 
blue- grey flags, and contain, besides the above-mentioned 
fossils, a Conocoryphe and a Bellerophon. The whole group is 
very barren of animal remains, and appears to have been 
deposited in shallow water. The well-known fossil, Cruziana , 
which is, I believe, an Alga, occurs abundantly near Bangor 
and in other areas in this group; and a large species of 
Buthotrephis was found in it by the late Mr. Belt. 
Upper, I 
50 ft. J 
Lower, 
2000 ft. 
Bluish-grey Flags, with Conocoryphe bucephala and Belle- 
rophon cambrensis , &c. 
Arenaceous and Micaceous Yellowish-grey Flags, with 
Lingulella Davisii, Hymenocaris vermicauda, Cruziana , 
&c. 
