Correspondence. 381 



respects agrees with, the characters of this genus, so far as they are 

 given in the Eeport. The variety of Biplodus with a single spine 

 also exhibits the same peculiar feature. 



The characters of the four other genera, described in the paper, 

 are too concisely given in the Eeport to enable me to form any 

 decided opinion respecting them, though I venture to think that 

 more than one are apparently only varieties of previously described 

 forms. 



Thomas Atthey. 

 Gasforth, Newcastle-on-Tyne, 

 Juli/ 15, 1867. 



THE WEAVER CLAYS. 

 To the Editor of the Gteologioal Magazine. 



Sib, — "When I paid my first visit to the Eibden Fire-clay Pit 

 several years ago, I formed pretty much, the same opinion as that 

 now held by Mr. Green, viz., that the deposit consisted of local 

 washings from surrounding strata, gathered into a wide fissure. 

 I have since then, however, made many journeys to the district, 

 and I have convinced myself, from a leisurely examination of all 

 the openings which have been made in search of "clay," that the 

 deposit is a very extensive one, and tndy of marine origin. The 

 beds extend over an area of nearly two miles in length, with a 

 width varying from a quarter of a mile to a mile and a half. 



Mr. Green is in error in attributing the parentage of the sands 

 and clays to the Bunter. The greater portion of the deposit has 

 certainly been derived from the Millstone grit strata that still pre- 

 vail so largely to the westward of Weaver. Immense blocks of 

 grit, of different degrees of hardness and coarseness, are thickly 

 embedded in the sands at Caldon Low, and they are to be seen in 

 every state of degradation, from the hard unperished stone to in- 

 coherent sand, that merely shows where the blocks previously 

 existed, by a slight difference of tint from that of the sand of the 

 matrix. A very small percentage only of the sands of the deposit 

 have been derived from the Bunter. 



The sands in some parts of the deposit are as white as the best 

 Alum Bay sand. They are so free from iron that, at my recom- 

 mendation, they have been tried lately for glassmaking, and they 

 have been found to answer very well. Bunter sand could scarcely, 

 under any circumstances, have been washed sufficiently free from 

 iron to stand this manufacturing test. 



With regard to the " Boixlder-clay Drift," I must state it to be my 

 firm conviction that the red bed which overlies the " Weaver Clays," 

 in some parts to the thickness of from twelve to fifteen feet, does 

 really belong to that formation. It lies in many places where it 

 could not have been deposited by subaerial action ; it contains 

 rounded and subangular masses of stone of many formations, and it 

 has itself been subjected to after-denudation. It is also of similar 

 character to the red clay that is seen to fill the fissures in Caldon 

 Low, at a height of twelve hundred feet; and to be piled on the 



