146 THE CARBONIFEROUS SYSTEM. 



that the two must stand or fall together. It also seems to me to 

 involve radical inconsistencies; for if I comprehend it, it asserts (1.) 

 That the flora of the whole coal measures (25,000 feet?) is identical; 

 that is, the vertical distribution of each and all the plants is complete 

 from the bottom to the top. (2.) Tbat nevertheless there are differences 

 observable between different coal-beds. (3.) That these are attributable 

 rather to difference of station and conditions of preservation than to 

 lapse of time ; that is, if we could take the beds, each one in its whole 

 extent and its fossils in their original condition, there would be no 

 differences observable between different seams after all. (4.) That 

 groups or assemblages of species in the Lower, Middle, and Upper 

 Coal measures may nevertheless be distinguished ; that is, while each 

 and every species may be found occasionally in all parts of the column 

 from bottom to top, yet this happens in such a manner as to group 

 some of them more abundantly, or in certain peculiar proportions in 

 the Lower, others in the Middle, and others in the Upper portions of 

 it. (5.) That, after all, however, these groups are not persistent, but 

 differ at different localities, and are as worthless as the specific forms 

 themselves for the identification of a single bed in more than one 

 place. — Is it possible that all this has been made out, or can be made 

 out, except in a country of horizontal coal measures, well opened for 

 study, where the stratification can be established beforehand, and the 

 range of the fossils be undoubted ? " 



With reference to this rejoinder, as Professor Lesley seemed to 

 have misapprehended some of the points briefly stated in my first 

 letter, I thought it necessaiy to make the following additional expla- 

 nations : — 



" 1. Dr Dawson is not aware that he has, at any time, maintained 

 that the " coal measures proper" of Nova Scotia are 25,000 feet in 

 thickness. In speaking of their enormous thickness, he referred to the 

 actual measurements of Sir W. E. Logan at the Joggins, which give 

 for the whole of the Carboniferous rocks seen in that section, a vertical 

 thickness of 15,570 feet, and for the coal measures proper, or Middle 

 Coal formation, a thickness of rather less than 10,000 feet. The 

 objections based by Mr Lesley on this supposed thickness of 25,000 

 feet, are therefore quite inapplicable to the views of Dr Dawson. 



" 2. Dr Dawson does not admit the interpretation of his views as 

 to the unity of the coal flora given by Mr Lesley. The 'inconsistencies' 

 alleged by the latter depend in part on the imaginary thickness of 

 25,000 feet attributed to the Middle Coal measures. The identity of 

 the flora throughout the Middle Coal formation, and the distinctions 

 between this and the assemblages of plants in the Lower and Upper 



