576 



2, Lower 

 Cambrian 

 Eocks. 

 (Long- 

 mynd 

 Group). 



ian 



TREMADOC SLATES. 



Harlech 

 grits. 



Sandstones 

 grit. 



and 



slates. 



6000 

 to 



7000 



Llanberis j Slat fv with ^ ) about ' 

 •< strata inter- V 



( mixed. ) 



3000 



[Ch. XXVII. 



Annelids, five spe- 

 cies (Arenicolites 

 sparsus, &c. ; one 

 crustacean ; Old- 

 hamia. 



LAURENTIAN GROUP. 



1. Upper Lauren tian, or Lab- 

 rador Series. 



2. Lower Laurentian. 



Stratified highly "") 

 crystalline rocks, j 

 with much La- 

 bradorite, and 

 other varieties of 

 felspar. 



Gneiss ; Quartzite") 

 Hornblendic and 



micaceous 

 schists, with 

 dense intercala- 

 ted limestones, 

 one above 1000 

 ft. in thickness. 



VI 2,000^ None, 



18,000 



Foraminifera {Eo- 

 zoon Canadense). 



UPPER CAMBRIAN. 



Tremadoc Slates. — The Trernadoc slates of Sedgwick are more than 

 1000 feet in thickness, and consist of dark earthy slates occurring 

 near the little town of Tremadoc, situated on the north side of Car- 

 digan Bay in Carnarvonshire. These slates were first examined by 

 Sedgwick in 1831, and were reexamined by him and described in 

 1846,'* after some fossils had been found in the underlying Lingula 

 flags by Mr. Davis. The inferiority in position of these Lingula flags 

 to the Tremadoc beds was at the same time established. The over- 

 lying Tremadoc beds were traced by their pisolitic ore from Tremadoc 

 to Dolgelly. No fossils proper to the Tremadoc slates were then 

 observed, but subsequently, when the same beds were well searched 

 by the collectors of the Government Survey in 1853 and 1857, thirty- 

 one species of all classes were found in them and determined by Mr. 

 Salter. By their means he was able to separate the beds into an 

 upper and lower, division: in the upper of which there are about 

 twenty species, and about fifteen in the lower. We have already 

 seen that in the Lower Llandeilo (Stiper-stones or Arenig group), 

 where the species are distinct, the genera agree with Silurian types ; 

 but in these Tremadoc slates, where the species are also peculiar, 

 there is about an equal admixture of Silurian types with those which 

 Barrande has termed " primordial." Here, therefore, it may truly be 

 said that we are entering upon a new domain of life in our retro- 

 spective survey of the past. The trilobites of new species, but of 



* Geol. Quart. Journ., vol. iii. p. 156. 



