XXXV111 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Tremadoc Slate. — Above the true Lingula-flags lie a set of strata 

 long ago named by Professor Sedgwick the Tremadoc Slate ; and late 

 researches have brought to light in these beds a series of fossils which 

 Mr. Salter has proved are mainly distinct from those of the Lingula- 

 flags below, and of the Llandeilo and Bala beds above them. Thus 11 

 genera of Trilobites occur in the Tremadoc Slate, which, excepting 

 4* (Dikelocephalus, Conocephalus, Olenus, and Agnostus), are generi- 

 cally distinct from those of the Lingula-flags. The species are en- 

 tirely distinct. At present the Pteropod Theca is first known in 

 these beds, together with Bellerophon, Conularia, Orthoeeras, and 

 Cyrtoceras ; and of all the forms not Trilobitic, Lingula Davisii and 

 L. lepis are the only species common to the two formations. 



Llandeilo and Caradoe Beds. — The community of the ordinary 

 species in the Llandeilo and Caradoe or Bala beds induces me to 

 treat them as one group, though it may be possible to subdivide 

 them in the cabinet, by help chiefly of certain Trilobites, into Lower 

 and Upper Llandeilo and Caradoe strata. The prodigious develop- 

 ment of fossils in these rocks has no parallel in the underlying 

 British formations, — a circumstance of great importance, especially 

 when we consider that they are to a very great extent new generi- 

 cally, and almost entirely specifically. But, notwithstanding this 

 sudden change, no visible unconformity has yet been detected where 

 (for instance, in the neighbourhood of Ffestiniog and Tremadoc) 

 the Lower Llandeilo beds lie directly on Tremadoc Slate. I shall, 

 however, presently show that such an unconformity may with reason 

 be suspected ; and though it is difficult, or perhaps impossible, to 

 separate Lingula-, Tremadoc, and Llandeilo beds (all chiefly slaty) in 

 mapping, it is not the less evident that in these strata we have three 

 perfectly distinct zones of organic remains, and therefore, in common 

 terms, three distinct formations, each much more definitely separated 

 by fossils from the others than, for instance, the different subdivisions 

 of the Wenlock and Ludlow beds, each of which can be mapped. 



Now all known evidence tends to prove that in Wales the Tremadoc 

 Slate is an exceedingly local formation, and for various reasons. It 

 is only certainly known in Merionethshire ; and in Pembrokeshire, 

 though carefully searched for by Mr. Salter, none of its fossils occur. 

 Neither is it clearly known in Caernarvonshire ; it is certainly absent 

 in Anglesey ; and in Scotland, rocks supposed to be Llandeilo flags lie 

 quite unconformably on supposed Cambrian strata. A fragmentary 

 formation, such as these Tremadoc beds seem to be, indicates one of 

 three things. 



1st. The formation may have been deposited over the whole area, 

 but was subsequently disturbed, and in great part removed by 

 denudation, before the deposition of the Llandeilo flags ; or, 



2nd. The Lingula-flags were partly upheaved before the deposition 

 of the Tremadoc Slate, and only those Lingula-beds that still remained 

 below water had Tremadoc slates deposited above them in direct 

 succession. 



* The remaining seven of these are Angelina, Asaphus, Cheirurus, Ogygia, 

 Ampyx, Psilocephalus, and Niobe. 



