Reports and Proceedings—Greological Society of London. 43 
Western or Criccieth District. Eastern or Tremadoe District. 
Rhyolitic ashes and agglomerates. 
Caraboc Variable dark - grey and _ black | Greyslate series, often strongly 
SERIES. shivery slates, banded, but with banded, with a few shelly 
no distinguishable horizons. fossils (Trinucleusand Orthis) 
in the upper part. 
Andesitic ashes and ashy shales. 
Dark banded slates with intense Vesicular andesites. 
cleavage ; no fossils found. Blue-black slates containing 
LLANDEILO graptolites. Zone of Nema- 
SERIES. graptus gracilis, 
Earthy slates, with occasional 
‘tuning-fork ’ graptolites. 
Shivery slates passing down into 
ARENIG Flaggy grits yielding Calymene 
Series. ) parvifrons. 
i Basal conglomeratic grit. Conglomeratic grit of Ynys 
Towyn. 
Unconformity. 
Garth Hill Beds: Not complete in the 
Grey-blue slates with Angelina. area studied. 
Penmorta Beds : 
Flagey mudstones and thinly-bedded slates, 
with Shwmnardia and the Shineton fauna. 
Portmadoce Beds : 
Thickly-bedded felspathic slates, with occa- 
sional Asaphellus. 
Moel-y-gest Beds : 
Banded grey slates and mudstones; few 
fossils, Acrotreta and Bellerophon. 
The Dictyonema Band: 
A constant and characteristic band of bright 
rusting blue-grey mudstones, with abundant 
Dictyonema sociale. 
Tynllan Beds: 
Thinly-bedded rusty shales with some hard 
grey mudstone bands, containing Viobe and 
Psilocephalus. (Symphysurus.) 
Sooty-black mudstones with Peltwrascarabeoides. 
Blue-black mudstone with Agnostus trisectus. 
Black slates, with calcareous bands often crowded 
with Orthis lenticularis. 
Dark flaggy slates with Parabolina spinulosa. 
, Grey-blue slates and flags crowded with Lingw- 
FFESTINIOG { lella davisii. 
TREMADOC 
SERIES. \ 
DoLGELiy { 
SERIES. 
—— 
SERIES. Grey flags and grauwacké with some coarser 
bands (1800 feet thick). 
Marntwroe ¢ Rusty grey and blue slates, with thin bands of 
Series. | felspathic grauwacke. 
The folding, the cleavage, the faulting, and the jointing of the rocks 
are described, and an attempt is made to show some relationship 
between the various stress-phenomena which have produced these 
structures. 
The great fault through Penmorfa is interpreted as a thrust-plane 
hading gently to the north-east. It is described as bounding two 
districts which are of a very different structural type, and is supposed 
to form the lowest sole of the group of thrust-planes which follow the 
southern margin of the Snowdonian mountain-tract. 
