XIV PERMIAN FOSSILS. 
condition, iternal evidence of having been deposited in a sea little affected by 
disturbing influences. It also appears to have been formed in shallow water; since 
the surface of some of its beds, between tide water marks, opposite Sunderland," is 
as distinctly rippled as the alluvial sand accumulated in the immediate vicinity. The 
entire absence of Corals, and Palliobranchiate shells in these beds, may perhaps be 
adduced as another argument in favour of the view which has been advanced: besides, 
such shells as Mytilus septifer, Schizodus Schlotheimi, &c., which they exclusively 
contain, might be instanced as another accordant evidence. 
The internal structure of the last-noticed member, constitutes a most important subject in lithological 
Geology. It may be safely stated, that few rocks in the entire series of stratified formations present such 
singular and varied aspects as are often displayed by the uppermost member of the Permian System in the 
County of Durham. Some of its beds are as different from what they were originally, as the most decided 
metamorphic rocks. This member, it has already been stated, consists of crystalline, earthy, compact, and 
oolitic limestones ; but it will be more convenient for present purposes to say, that it consists of crystalline, 
and non-crystalline limestones. The former are largely developed at Building Hill, and several other places ; 
the latter, at Hartlepool, and some other localities on the coast of Durham. The crystalline kinds consist 
chiefly of carbonate of lime; but the non-crystalline (oolitic), contain a large portion, as much as 
44 per cent. (Johnston), of carbonate of magnesia in addition. Various hypotheses have been proposed to 
account for the presence of so large a quantity of carbonate of magnesia in the non-crystalline limestones : 
it is the general opinion, however (and I am quite in favour of the same view), that this substance is an 
original constituent of the rock, or, in other terms, that it was deposited at the same time as the carbonate 
of lime with which it is associated.2 But the most singular circumstance connected with this member is, 
that the same bed is often crystalline and essentially calcareous in one part, and non-crystalline and mag- 
nesio-calcareous in another not a yard apart: nay, hand specimens display precisely the same molecular and 
chemical differences. In some localities, the crystalline hmestones assume the most singular appearances, 
consisting of enormous branching radiating coral-like masses, or of globular, hemispherical, discoidal con- 
cretions, the latter varying from the size of a marble to that of the largest cannon-ball. The cause of these 
singular structures has ever been, and will probably long remain, a perplexing problem. None of the 
hypotheses hitherto advanced to account for them, appear to me to be satisfactory: this is considered a 
sufficient reason for my attempting to emulate previous writers in speculating on their origin. 
Like most sedimentary rocks, the Protozoic especially, many of the beds of the Permian system, in ad- 
dition to their natural partings, or those displayed on their lines of deposition or stratification, are very 
much and continuously intersected or divided by cleavage splits, varying extremely in their distance from 
each other. Now in brief terms, although the connexion is completely obscured in some places, I have 
generally found, that wherever the coralloidal, or globulo-concretionary structures are most distinctly 
developed, it is in the immediate vicinity of either the cleavage splits, or the deposition partings. A few years 
since, at Building Hill, near Sunderland, the quarrymen exposed several beds, which were completely divided 
into great irregular rhomboidal blocks, the form of which has resulted from the cleavage splits passing con- 
tinuously and somewhat obliquely through the beds. In this particular instance, there were displayed the 
most singular and beautiful coralloidal forms I have ever seen. When one of these rhomboidal blocks was 
broken, it disclosed a number of dark brown radiately-branching coralloidal bodies striking off towards the 
1 This locality, which was immediately opposite the old battery, is now occupied by the new docks. 
2 Dr. Richardson is inclined to ascribe the formation of the constituents of the magnesian limestone 
“to the influx of waters holding chloride of magnesium in solution, which, meeting with calcareous matter 
held in solution by an excess of carbonic acid, robbed it of that excess, and the two carbonates of lime and 
magnesia fell together. (Vide Report Brit, Association, 1842, part ii, p. 37.) 
