English Stratified Rocks inferior to the Old Red Sandstone. 309 



and degree. The mean intensity can therefore only be collected 

 by ascertaining the intensity during every geological period, and can 

 never be obtained by assuming the intensity of any one e^joch, jjast 

 or present, as the arbitrary standard. Again, the successive organic 

 types indicate great physical changes ; and following the descending 

 scale they at length vanish ; conducting us, however, to the confines 

 of other investigations in exact science which must prove the ulti- 

 mate basis of physical geology. 



Finally, the author gives a tabular arrangement of the several 

 classes and subdivisions agreeably to the system of the preceding 

 communication. 



Class I. — Primary stratified Groups. 

 Gneiss, mica slate, &c., &c. Highlands of Scotland and the 

 Hebrides. Crystalline slates of Anglesea and the S.W. coast 

 of Carnarvonshire. 

 The series generally without organic remains; but should organic 

 remains appear unequivocally in any parts of this class, they may be 

 described as the Protozoic system. 



Class l.(«.) The crystedline slates of central Skiddaw forest, and 

 the upper Skiddaw slate series. The whole is inorganic and inter- 

 mediate between Class I. and Class II. 



Class II., or Paleozoic series. 

 This class Includes all the groups of formations between Class I. 

 and the old red sandstone ; and is subdivided as follows : — 



1. Lower Cambrian System. — All the Welsh series under the Bala 



limestone. The two great groups of green roofing slate and 

 porphyry on the north and south side of the mineral axis 

 of the Cumbrian mountains. A small part of the slates of 

 Cornwall and South Devon. ? A part of the slate series of 

 the Isle of Man, &c., &c. 



2. Upper Cambrian Sy stein. — A large part of the Lammermuir 



chain on the south frontier of Scotland. A part of the third 

 Cumbrian group, commencing with the calcareous slates of 

 Coniston and Windermere. The system of the Berwyns and 

 South Wales. The slates of Charnwood forest. } All the 

 North Devon and a part of the South Devon series. The 

 greater part of the Cornish series. 



3. The Silurian Systejn. — The upper part of the third Cumbrian 



group, chiefly expanded in Westmoreland and Yorkshire. The 



flagstone series of Denbighshire. The hills on both sides 



of Llangollen. The region east of the Berwyn chain. The 



regions described in the papers of Mr, Murchison, from which 



the types of the system are derived. The lowest part of the 



culmiferous series. ? 



Over all the preceding comes the Old Red Sandstone — divided into 



three great natural gromps in the country bordering the Silurian 



types of Mr. Murchison ; in the northern counties developed in a 



less distinct manner, chiefly in the form of great unconformable 



masses of conglomerate, appearing at irregular intervals between the 



preceding groups and the carboniferous series. 



