802 Geological Society: Prof. Sedgwick on the 



that have brought up a portion of the older rocks (of the class here 

 described) at Dudley, on both sides of the Warwickshire coal field, 

 and in Charnwood forest. At all these localities the strike is the 

 same, and the lines of greatest movement are nearly parallel — all being 

 about N.N.W. and S.S.E. ; and all these movements belong to one 

 epoch, having been completed after the deposition of the lower red 

 sandstone, and before the period of the upper and gypseous marls. 

 Hence we have three great systems of elevation, each marked by 

 parallel lines of strike, and the three systems of strike indicating 

 three distinct periods of elevation. 



The author then points out the importance of such facts to the 

 broad speculations of geology, as well as the limitations under which 

 they are to be applied. The dynamical powers of elevation appear 

 to have been employed in three principal forms. 1st. In gradually 

 raising up ridges through large spaces of the earth's crust. These 

 will explain the correspondence of strike through very extensive 

 regions ; and such elevations if continued beyond a certain limit must 

 have produced longitudinal fissures and lines of volcanic vent. 2ndly. 

 In the long-continued protrusion and eruption of igneous rocks along 

 such lines of vent. 3dly. In local and partial eruptions and pro- 

 trusions, producing valleys of elevation, local derangements, and other 

 phsenomena that terminate in ordinary volcanic action. Elevatory 

 forces, when considered in this general way, explain the phaenomena 

 of strike — the parallelism of great contemporaneous elevations — as 

 well as the exceptions to the rule of parallelism. 



GROUPS OP THK CUMBRIAN SECTION, &C. 



The author then commences the separation of the whole series of 

 rocks of the second class into natural groups, founded on sections ex- 

 hibited in the several districts above noticed ; and after shortly dis- 

 cussing two sections connecting the Cheviot bills with the formations 

 in the basin of the Tweed, he describes in some detail a transverse 

 section through the whole system of the Cumbrian mountains, whieh 

 exhibits the following groups in ascending order. 



(1.) The group of Skiddaw Forest, &c., the lower part of 

 which rests on the granite, and passes into a system of crystalline 

 strata resembling the rocks of the first class ; the upper part abounds 

 in a fine dark glossy clay slate, interrupted here and there by beds 

 of more mechanical structure. The whole is of great thickness, al- 

 most without calcareous matter, and without any trace of organic 

 remains, and forms the mineral axis of the Cumbrian mountains. 



(2.) A group essentially composed of quartzose and chloritic roof- 

 ing slates alternating with mechanical beds of coarser structure, and 

 also with innumerable igneousrocks (compact felspar,felspar porphyry, 

 brecciated porphjnries, &c. &c.,) which partake of all the accidents 

 of the slates. It is of enormous thickness, and rises into the highest 

 mountains of the country ; and though chiefly developed on the 

 south side of the preceding group (No. 1), it also appears extensively 

 on the north side of the lower group, which thus forms a mineral axis 

 «— a fact not yet noticed in any of the published geological maps. 



