300 On Coal Forma ilons. 



each other, hi crossing^ at the focal point of a lens or specuhnn, 

 as to be neutralized, and thereby lose the property of acting 

 upon the retina? R, B. 



LIV. On Coal Formations. By W. H. Gilby, Esq. 



To Mr. Tilloch. 



^iB., — X HE following remarks have originated from a paper of 

 Mr. Farev's in the last number of the Philosophical Magazine, 

 res]iecting the coal formations in North Wales, and the relation 

 of the red ground to the coal and its accompanying strata. From 

 tlie general tenor of this paper, Mr. Farev seems to claim the 

 ere<lit of Iiaving first brought forward to the notice of the public, 

 tiie fact that the different beds of the red ground so widely distri- 

 buted through many parts of England, lie in an unconformable 

 position over the inclined coal measures. If that gentleman had 

 consulted a description of the neighbourhood of Bristol, which 

 you did nie the favour of inserting in one of your late numbers, 

 he u'ould have found that this circumstance had been then most 

 explicitly detailed. T!;e arrangement wliich for the sake of 

 method and clearness I followed in that paper was entirely founded 

 upon this very circumstance. For as the formations in the neigh- 

 bourhood of Bristol con'3ist either of inclined strata or of such 

 as lie " unconformably in an horizontal position upon the tops of 

 the inclined strata;" 1 began by describing those of the first de- 

 scription, comj)rehendiT)g the inclined coal measures, the moun- 

 tain limestone, and other rocks which dip under them. I then 

 proceeded to treat of tiie iincori formal le horizontal series, be- 

 ginning from the beds of the red marl and passing upwards to 

 the lyas, &;c. As this account was printed in a journal which 

 Ivlr. Farey is in the habit of making the repository of his prac- 

 tical observations, I cannot suppose that he was unacquainted 

 T.itii it ; and as he bus alluded to no luriler who had noticed the 

 fact before him, he plainly wishes to appear as the first an- 

 nouncer of it. It is very evident, therefore, that Mr. Farey has 

 acted in an unfair and disingenuous manner in manoeuvring for 

 himself a gloriola, which he does not deserve ; and whatever con- 

 sequence may be attached to the observation, — and I elearlv see 

 that it is of considerable importance to coal proprietors, — I shall 

 at least be entitled to the priority of having ascertained it before 

 Mr. Farey, inasmuch as my paper was printed full half a year 

 before his. — 1 cannot suppose that Mr. Townsend is ignorant of 

 the relation which the red ground bears to the coal measures ; 



but. 



