Devonian System. 5 1 1 



Mr. Greenough, in the new edition of his map of England, repre- 

 sents nearly the same boundaries and order of succession in Devon 

 and Cornwall as we find in the maps of Mr. De la Beche and Messrs. 

 Sedgwick and Murchison ; but in his memoir connected with the 

 map, adopting the name of Carbonaceous series for the culmife- 

 rous rocks, he substitutes that of Upper killas for the Devonian 

 system of Sedgwick and Murchison, (including under that term 

 the old red sandstone of Herefordshire,) and Lower killas for the 

 slates inferior to the Silurian system, which they have termed Cam- 

 brian. 



Mr. Greenough, in his memoir, also shows by quotations from Dr. 

 MacCulloch, that the undisputed old red sandstone of the north of 

 Scotland exhibits, at intervals, the same great changes of mineral 

 character, that occur in the strata intermediate between the Carbo- 

 naceous and Silurian systems in the west of England and on the 

 borders of Wales; and justly infers the inadequacy of any one term 

 to characterize formations which vary so much in lithological com- 

 position, that at one place they present the condition of a fine- 

 grained silky slate, at another of sandstone, and at a third that of 

 coarse gravel and conglomerate rock. 



Thus, with respect to the slate rocks of Devon, Cornwall and 

 Wales, the difficulties are reduced to those of an unsettled nomen- 

 clature ; whilst nearly all parties are in unison as to the fundamental 

 fact of referring the slates of South Devon and Cornwall to the epoch 

 of the old red sandstone formation. The term grauwacke, however, 

 I rejoice to think, will not be condemned to the extirpation which 

 has been threatened from the nomenclature of geology; it may still 

 retain its place as a generic appellative, comprehending the entire 

 transition series of the school of Freyberg, and divisible into three 

 great subordinate formations: the Devonian system of Sedgwick 

 and Murchison being equivalent to the upper grauwacke, the Si- 

 lurian to the middle grauwacke, and the Cambrian system to the 

 lower. 



In this threefold distribution of the vast series of strata which 

 have hitherto been indiscriminately designated by the common term 

 grauwacke, we are, as it were, extending the progressive operations 

 of a general inclosure act over the great common field of geology ; 

 we propose a division, founded on measurements, surveys, and the 

 study of organic remains, analogous to that of the secondary strata, 

 from the chalk downwards to the coal formation, established by 

 William Smith, and to the separations of the once undivided ter- 

 ritory of the great tertiary system, effected by Cuvier and Brongniart, 

 Desnoyers, Lyell, and Deshayes. 



To the uninitiated in geology, rectifications in the distribution 

 of strata upon so large a scale may seem calculated to shake confi- 

 dence in all the conclusions of our science ; but a contrary inference 

 will be drawn by those who know that these corrections have never 

 been applied to conclusions established on the sure foundation of 

 organic remains, but to those rocks only of which the arrangement 

 had been founded on the uncertain character of mineral compo- 

 sition. 



