862 



Prof. Sedgwick on the May Hill Sandstone, 



A Tabular View of the Primary or Pakeozoic System of England 

 and Wales, separated into three principal divisions, — Lower, 

 Middle, and Upper. 

 I. Lower Division, representing the Cambrian and Silurian 

 series in ascending groups. 



Longmynd slates, &c.* 

 Llanberris slates; alterna- 

 tions of roofing-slates and 

 grits. 

 Harlech grits; sometimes 

 approaching a conglome- 

 rate form. 



1. Longm5'nd and Bangor group ^ 

 (Lower Cambrian) 



Festiniog group . . . . ^ 

 (Middle Cambrian)^ 



Bala group 



(Upper Cambrian) 



a. Lingula flags. 



b. Tremadoc slates. 



c. Arenig slates and porphyries, &e. 

 Slates, flags, and grits; indefinite 

 alternations of porphyry and trap- 

 shale : one irregular band of lime- 

 stone near the top of the group. 



a. Lower Bala rocks. (1) A great zone 

 of dark and sometimes earthy slate. 

 (2) A great series of slates, flags, and 

 grits, ascending to the Bala lime- 

 stone. 



b. Upper Bala rocks. Under this term 

 are included the Bala and Himant 

 limestone, the Llandeilo calcareous 

 flag, and the shelly sandstones of 

 Caer Caradoc. The whole series 

 ending, in North Wales, with slate 

 and flagstone ; and, in South Wales, 

 with slates, grits, and coarse conglo- 

 merates. 



Immediately above these three groups there is a great change 

 of physical conditions. The most characteristic old organic 

 types disappear, and new types take their place. The sections 

 are usually broken and discontinuous ; and the upper (or Silu- 

 rian) groups sometimes overlap the lower (or Cambrian) groups. 

 Here, therefore (to adopt a language in common use), we have 

 the commencement of a new system. 



* The Lon^ynd slates are here arranged ynth the Bangor group, partly 

 on the authonty of the Government Survey. The other Cambrian groups 

 are exclusively from my own observations. In a letter, pubhshed as an Ap- 

 pendix to a Guide to the Lakes (Hudson, Kendal, June 1853), I have used 

 the following words : — ** After the granitic rocks of Wales, we have Meta- 

 morphic and Hypozoic rocks. The Metamorphic, of a great, but some- 

 what doubtful age, and of great thickness. The Hypozoic rocks (of 

 Longmynd, &c.) of very great thickness, and also of doubtful age, but 

 probably to be linked to the lowest Cambrian groups, and to be placed on 

 the general parallel of the Skiddaw slates. If so, they will cease to be 

 Hypozoic, and may then be considered as the lowest known base of the 

 Cambrian series. 



