444 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



stating and testing the opinions of the gifted brothers we have just 

 named, who have most successfully devoted their great intellects in 

 life-long labours to the study of physical geology, and under the 

 most favourable circumstances. 



As some apology for differing with geologists of the first order, 

 some of them perhaps less familiar with quiet than with disturbed 

 districts, I may be permitted to say, that it is unfaithful to scientific 

 progress, and to the sacredness of truth itself, to rest too exclusively 

 on authority, — not, after unbiased study and inquiry, to write mo- 

 destly what we think honestly. Neither do I stand alone. 



In our examination of the different groups of the Lower Silurian 

 stage, we have seen a greater or less connexion between them, and 

 commonly both as to fossils and mineral character ; but on the 

 complete deposition of the Hudson-River rocks new conditions su- 

 pervene ; and all observers are agreed that here is placed a dis- 

 memberment — a cessation, in degree \ of palaeozoic sequences, and 

 therefore a new starting-point. 



So far all opinions coincide ; but opposite sides are taken, and main- 

 tained with honourable pertinacity, as to whether this stop or break 

 is a break of system or of stage ; in other words, whether or not all 

 below the Oneida Conglomerate of Middle North-east America should 

 be denominated Cambrian^ and all above it Silurian. 



If the two propositions stated in page 38 be true, as I believe 

 they are, the limited change, physical and vital, which they admit, 

 at this period, is reduced to the inferior significance and value of a 

 break of stage only. If, however, with Professors Rogers, Sedgwick, 

 and other eminent men, we see a wide extending crust-rupture of 

 almost unparalleled force and range, at the period of the Oneida Con- 

 glomerate, by which utter destruction fell upon existing surfaces, fol- 

 lowed by a totally new series of deposits and their inhabitants, then 

 this break is to be deemed systematic. 



Before exhibiting more fully the views of these gentlemen, I beg 

 to observe that they appear to have formed too low an estimate of 

 the change of all conditions implied in the establishment of a system. 



A new system at any epoch is not necessarily created by a mode- 

 rate change of levels, by the presence of conglomerates, or by the 

 extinction of a fauna. Were this the case, systems would almost 

 equal in number the Silurian sections themselves. 



Much more is required. If we pass from one system to another, 

 as from the Silurian to the Devonian (which is a mild instance), 

 we find a new phase, new actions, and new animal forms in the 

 latter, and these of great import. Little physical disturbance has 

 taken place. Dry land has increased in quantity ; and we have a 

 terrestrial vegetation for the first time. The molluscs have a new 

 facies, are very much larger, more ornamented, and rather more 

 numerous than in Silurian sediments. Few of the old forms are left ; 

 many new and singular animals come forth ; and, what in scientific 

 value exceeds all other considerations, a novel and completely dif- 

 ferent type of life is introduced — the vertebrate (in the forms of 

 fishes and reptiles) — a type which, after many and constantly ascend- 



