Geological Society. 



205 



Tremadoe, and Criccieth in Carnarvonshire, and describes the 

 stratigraphy of the Cambrian and Ordovician rocks there exposed. 

 The area described includes the original type-area of the Tremadoe 

 Slates (Sedgwick & Salter), and the paper includes a detailed 

 account of the local development of this well-known series. 



The first part of the paper is devoted to a brief summary of the 

 results attained by former workers, and is arranged to show the 

 various stages through which the nomenclature of the major 

 subdivisions of the Cambrian and Ordovician Systems have evolved. 



The sedimentary series are described in the order of their 

 formation. The succession may be tabulated as follows : — 



Caradoc 



Series. 



Llandeilo 

 Series. 



A ren to 

 Series. 



Western or Criccieth District. 



Ehy otitic ashes and agglomerates. 



Variable dark-grey and black 

 shivery slates, banded, but. with 

 no distinguishable horizons. 



Dark banded slates 

 with intense cleavage ; 

 no fossils found. 



Earthy slates, with occasional 

 'tuning-fork' graptolites. 



Shivery slates parsing down into 

 Flaggy grits yielding Calymene 



parvifrons. 

 Basal conglomeratic grit. 



Eastern or Tremadoe District. 



Grey slate series, often strongly 

 banded, with a few shelly 

 fossils (Trinucleus and 

 Orthis) in the upper part. 



Andesitic ashes and ashy shales. 

 Vesicular andesites. 



Blue-black slates containing 

 graptoli tes. Zone of Nema- 

 graptus gracilis. 



Conglomeratic grit of Ynys 

 Towyn. 



Tremadoc 

 Series. 



Unconformity. 



Garth Hill Beds: Not complete in 



Grey-blue slates with Angelina. the area studied. 



Penmorfa Beds: 



Flaggy mudstones and thinly-bedded slates 

 with Shuimrdia and the Shineton fauna. 

 Portmad oc Beds : 



Thickly-bedded felspathic slates, with occa- 

 sional Asaphellus. 



Moel-y-gest Beds. 



Banded grey slates and mudstones ; 

 few fossils, Acrotreta and Bellcrophon. 

 The Dictyonema Band : 



A constant and characteristic band of 

 bright rusting blue-grey mudstones, with 

 abundant Dictyonema suciale. 

 Tynllan Beds: 



Thinly-bedded rusty shales with some hard 

 grey mudstoue bands, containing Niobe 

 and Pyocephalus. {Symphymrus.) 



