ROBERTS — DISTRIBUTION OF CEPHALASPIS AND PTEEASPIS. 189 



DISTRIBUTION OF CEPHALASPIS AND PTERASPIS. 



(Geol vol IV., p. 102 ; Ibid., p. W) 



g 1Rj — Before I trouble you with a few remarks in elucidation of the 

 apparent difference between the views of Mr. Lightbody and my own, 

 permit me to express the pleasure I feel at seeing his name so promi- 

 nently in your columns. No man has studied with such untiring 

 zeal the range and sequence of those interesting deposits which make 

 up the Ludlow promontory ; and I am glad to find that my own 

 investigations, carried on independently of his, and at a distance 

 from the field of work, have provoked so slight a bill of exceptions, 

 and a few interesting notes, which I am sure every reader of " The 

 Geologist" would be glad to see continued. 



The head and front of my offending, as I learn it from my friend's 

 comments is this : — I have called the " Passage-beds " Lower 

 Tilestones ; and out of this some confusion has arisen in minds 

 which associate the word "tilestones," upper or lower, with those 

 originally so-called, but known by Sir Roderick Murchison now as 

 the Downton-beds. To make the position of Mr. Lightbody and 

 mvself quite clear to your readers, I will refer them to my section on 

 p." 104, They atiII see that above the " Downton Sandstones" lies a 

 zone which I have called " Lower Tilestones," and which is marked 

 out by the number and variety of its fish-fossils. These beds are the 

 " Passage-shales" of Murchison, which have been so industriously 

 worked in their exposures in the Ludlow district (the chief of which 

 are in the railway cutting near the station, and at the Tin Mill, about 

 a mile distant) by my valued friend, Lightbody. 



But now followeth my reason for not, in my humble sketch of 

 ancient ichthyic life, retaining a name which has such high sanction . 

 I rejected the term because it appeared to me to have less value as 

 a designation for a special zone of deposit than the one I employed. 

 The horizon of a "Passage-bed" must necessarily, from the character 

 of the powers employed to deposit it, be a shifting one. True, that 

 no name or term of designation we can apply to any rock, or zone of 

 deposit, will be cosmopolitan in its value; but "Passage-beds" so 

 called, have more troublesome equivalents than deposits nearer to the 

 centre of a system, and in any endeavour to sketch out the range of 

 life-remains, the term seemed to me peculiarly inappropriate. Upon 

 this view of the case, I included all the beds beneath the " Upper 

 Tilestones" of Trimpley — and as I still think of the Downton Hall 

 drive quarry — under one name, as Lower Tilestones, representing 

 them as resting upon the Downton series of sandy and "tiley" rocks. 

 One good characteristic of the "Upper Tilestones" is their possession 

 of an intercalated plant-bed, with good evidences of terrestrial vege- 

 tation. This, it must be remembered, is quite a distinct thing from 

 the Downton plant-bed, which contains the earliest land-plants. The 



