xiv PERMIAN FOSSILS. 



condition, internal 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 Scldotlieimi, &c., which they exclusively 

 contain, might be instanced as another accordant evidence. 



The internal structure of tbe 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 qf 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 whicb it Is associated. ^ 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 limestones 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 



' This locality, which was immediately opposite the old battery, is now occupied by the new docks. 



2 Dr. Eichardson 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 Ume and 

 magnesia fell together. (Vide Report Brit. Association, 1842, part ii, p. 37.) 



