XXXVI PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



city of spirit. So, after arranging all his affairs and putting his 

 house in order, as if he were preparing for the great change, he went 

 abroad in the autumn of last year, and, after trying a few palliative 

 remedies in the waters of Germany, he died, at Lausanne, on the 

 8th of November, in the 67th year of his age. 



I propose now to discuss the 



Breaks in Succession of the British Paleozoic Strata. 



By breaks in succession I understand those physical interruptions 

 in stratification marked by the unconformity of an upper formation 

 to one immediately underlying it, or, when such visible unconformity 

 is wanting, by a sudden change in the fossils characteristic of the 

 underlying and overlying formations ; and I therefore only apply my 

 argument to those cases in which the upper formation is next in 

 time to that which underlies it, according to our present knowledge 

 of the order of succession. 



In these remarks I lay no claim to originality for the facts re- 

 corded, which are part of the common stock of knowledge of geolo- 

 gists ; but I am not aware that, in any special memoir, they have 

 ever been prominently classified. The train of reasoning founded 

 thereon has also, I suppose, been floating more or less in many 

 minds. With me it is a result of long personal knowledge of pheno- 

 mena in the field, and of the memory of what I have thought, read, 

 and learned in conversation and debate ; and as, in writing what fol- 

 lows, I have only consulted books when my memory seemed at fault, 

 if any one finds ideas to which he thinks he can justly lay claim, I 

 beg he will believe I have borrowed from him. 



Before launching into the subject I have only to add that the 

 species estimated as common to any two formations are from tables 

 compiled by myself, partly from Morris's catalogue and others in 

 various memoirs, and partly, for the Silurian and Devonian rocks, 

 from data furnished by Mr. Salter, and, for the Permian strata, by 

 Mr. Davidson. It will not materially affect my argument should these 

 estimates not be always strictly correct. For a long time palaeonto- 

 logists were busy generally in subdividing, but are now chiefly 

 engaged in condensing the number of described species ; and though 

 it is to be hoped they may ultimately diminish the number of 

 names, and though it is probable they may prove that more are 

 common to various formations than published lists now show, yet it 

 is probable that the mass of the species will still be considered as 

 essentially distinct in any two formations in which I state that they 

 are so, according to the common interpretation of the term species. 

 But constant varieties will do quite as well for my purpose. 



Laurentian Gneiss. — First, then, I begin with the more ancient 

 rocks of the North Highlands, and in doing so I treat the order of 

 superposition proposed by Sir Roderick Murchison as completely 

 established ; it happens that the question forms part of my argu- 

 ment, and I therefore take this opportunity of again bearing testi- 



