OF THE COUNTIES OF NORTHUMBERLAND AND DURHAM. 2216 
ter locality and at Tunstall they occasionally are found in a 
powdery substance, which fills the cavities between the harder 
portions of the rock. In this situation they are often finely pre- 
served, and can frequently be so entirely extricated from the en- 
veloping matter as to shew both the external appearance and 
internal structure. 
In some of the specimens from the thin crystalline bed at 
Byers’ Quarry and in some of those from Tunstall-hill, the ori- 
ginal structure of the shells has become crystalline. This change 
must have taken place at the time when the rocks in which 
they are imbedded, assumed their present appearance, for they 
possess the same cleavage and are oftentimes the nucleus of a 
radiated crystalline mass. Though this renders it extremely 
difficult to extricate the shell from its matrix, yet the original 
form of the shell has not been in the least injured nor the mark- 
ings on the outer surface obliterated. 
It has been supposed, from the comparative scarcity of marine 
remains found in these rocks, that many species of mollusca, 
fishes, etc. existed during this period and were afterwards im- 
bedded in the sediment, of which the different beds are formed, 
but owing to the earthy nature of the surrounding matrix 
and to the crystalline movements which took place after their 
deposition, have entirely disappeared.* From the observations 
which have just been made on the state of these fossils, it 
appears that they are found in some parts of this deposit, which 
are quite earthy and pulverulent as well as in those of a concre- 
tionary and highly crystalline structure. Indeed shells are pre- 
served in some of these rocks under conditions apparently so 
unfavourable to their preservation as entirely to prevent a conclu- 
sion being drawn that any have wholly disappeared for want of 
& proper matrix to secure them. 
It is worthy of remark that most of the species which are found 
in the compact limestone of Whitley, etc., occur also in some of 
the thin crystalline beds along the coast, in the concretionary 
beds of Marsden Hill, and in the fossiliferous bed at Humble. 
ton, etc., from the lowest to the highest fossiliferous beds of the 
Sedgwick, Geol. Trans. 2nd ser. iii., p. 99 . Ansted’s Geology, i., p. 236. 
VoL. I. Hh 
