314? Geological Society: — Prof. Sedgwick and Mr. Murchison 



5. The last group in this part of the ascending section commences 

 on the south side of a line drawn from Baggy Point on the coast in 

 the direction of the strata, or east and by south. It is composed of 

 arenaceous flag stones and soft earthy slates, alternating with harder 

 and coarser bands : it conforms to the mineral type commonly found 

 in the lowest part of the Silurian system, has abundance of organic re- 

 mains, and is in parts calcareous ; but the fossils are often ill pre- 

 served and partially destroyed by the cleavage passing through them. 

 The group is several thousand feet thick, and though much contorted 

 (the anticlinal and synclinal lines coinciding with the strike) at 

 length dips regularly under the base of the Culm Measures. Such is 

 the succession in this portion of the section across Devon. Three di- 

 stinct groups of calcareous and fossiliferous slates, separated from 

 each other by deposits of vast thickness, very little calcareous, and 

 almost without fossils. The base of the series is not exposed, and 

 the last ascending term conforming to the type, and probably of the 

 date of the lowest portion of the Silurian system. 



These last-mentioned rocks much resemble the lowest Silurian 

 strata of Pembrokeshire, which also graduate into the Cambrian 

 system, and in which the specific character of the shells is often ob- 

 literated or obscured by transverse lines of slaty cleavage. Impres- 

 sions of crinoidal stems abound ; trilobites occur but rarely, and 

 among the shells are two or three which cannot be distinguished from 

 lower Silurian fossils. As however no fixed line of demarcation has 

 yet been established between the lower Silurian and upper Cambrian 

 groups, their zoological contents being, as far as we know, very simi- 

 lar, the place of this member of the Devon series must, for the present, 

 be considered provisional. 



The sandstones of this division are in one district pretty abun- 

 dantly charged with impressions of plants, for an acquaintance with 

 which the authors express their obligations to Major Harding and the 

 Rev. D. Williams, both of whom have sent collections to the Geolo- 

 gical Society. Professor Lindley is of opinion that none of these 

 plants are similar to those described in the sequel as common to the 

 Culm Measures : some resemble decorticated Lepidodendra, and 

 others Sternbergia?. One specimen resembles Calamities Voltzii of 

 the Terrain d'anthracite inferieur (Voltz). 



The authors conclude their remarks on the whole region by some 

 account of joints. % 



Dip joints and strike joints abound in all the groups, and though 

 not constant in their inclination are generally inclined at a high 

 angle, separating the great masses into rhombohedral solids. The 

 transverse cleavage planes are not parallel to joints, and are regarded 

 by the authors as forming a distinct class of phenomena. 



Chap. III. — Deposits of the fourth region. 



The natural groups are determined by help of sections ; one from 

 Dartmoor to the coast of Torbay ; another from Torbay to Start 

 Point ; and a third from Dartmoor to Bolt Head. In describing 

 these sections the authors enter on many details not given in this 

 place, and from a review of the whole division, the following groups, 

 beginning, as before, with the lowest. 



