1857.] on the Malvern Hills. 393 



the arc to the length of the chord— these being more violent 

 towards the sides of the basin. Thus, in the central part of the 

 great basin of South Wales, the folds are few and very broad ; 

 but on the north the Vale of Towy, and on the south the Cliffs of 

 North Devon, are traversed by many very steep and complicated 

 undulations of the strata, which seem to be axes of violent move- 

 ment, and yet are, perhaps, realli/ the result of gradual lateral 

 thrusts occasioned by vertical movements on parallel lines at some 

 distance — movements which it is the business of a general theory 

 to explain. 



The phenomena of the succession of life in the Malvern district 

 were also treated of in this discourse, and illustrated by diagrams 

 representing the increase of variety of specific and generic forms 

 (not of the number of individuals)^ in different geological ages ; the 

 summary being to the effect that a curve of life may be drawn for 

 the north temperate zone, the ordinates of which are reciprocals of 

 the abscissae, or in other words, the variety of the forms of life, 

 beginning from zero in the hypozoic strata augments with the lapse 

 of time, toward the most recent strata. The curve has, however, 

 several points of contrary flexure, the most remarkable being two, 

 viz., first, at the close of the Palaeozoic period ; and secondly, at the 

 close of the Mesozoic series ; at which times also occurred very 

 great and general changes of physical geography, accompanied, no 

 doubt, by the drying of some oceanic basins, and the extinction of 

 their peculiar forms of life, and the opening of others to seas in 

 which new races had begun to take their place in the diversified 

 system of nature, 



[J. p.] 



