GEOLOGY 



in the numerous quarries inland. Its maximum thickness is about 800 

 feet, and this is attained beneath the red sandstones of Seaton Carew, as 

 proved by borings made at that place in 1888. Its minimum is in the 

 neighbourhood of Naughton, where it has been proved, also by boring, 

 to be less than 300 feet, but as there is a suspicion of the upper portion 

 of this formation having been denuded off at this spot this minimum 

 thickness is less certain than the maximum quoted. As the Marl Slate 

 is without doubt identical with the Kupf ers chief er so is the Magnesian 

 without doubt the equivalent of the continental Zechstein. Its curiously 

 stunted forms of peculiar marine fossils represented by many individuals 

 but comparatively few species are the same as those of the Zecbstein. Its 

 general but varying- dolomitic character, to which it owes its English 

 name, is the same ; and its position in the stratigraphical sequence is also 

 the same. In Durham however its lithological features are extremely 

 peculiar. Long after the limestone was deposited molecular movements 

 took place within the already consolidated rock which, in many places 

 and at many horizons, gave rise to a quite unique development of concre- 

 tionary structures. From the time of Sedgwick, who first described 

 them from a scientific point of view, to the present day when Dr. George 

 Abbott of Tunbridge Wells has spent the leisure intervals of many years 

 in studying and photographing them, the concretions referred to have 

 attracted and have puzzled geologists. They have been classified 

 according to their endlessly diversified forms, but the cause of so 

 much structural rearrangement in this formation has not yet been 

 clearly established. Professor E. J. Garwood has shown with regard 

 to the simpler spheroidal forms (which are known as the cannon ball 

 limestone) that these are due to the segregation towards centres of the 

 carbonate of lime previously existing in the rock, and not to the intro- 

 duction of that compound into the magnesian beds from without (this 

 latter was the so-called ' stalactitic theory ' of the late Mr. Richard 

 Howse), but it cannot be said that this, which is probably now admitted 

 by all, carries us very far. It is a theory accounting for the multi- 

 form character of the concretions, the ' honeycombed,' ' coralloid,' 

 ' oolitic,' ' botryoidal,' ' egg and cup,' and others infinitely varied besides 

 the spheroids that is required, and this probably experiment only will in 

 time provide. 



The Geological Survey in its maps has unfortunately not attempted 

 to divide the Magnesian Limestone. The task, owing to the extraordi- 

 nary variability of the rock now earthy, now flaggy, over and over 

 again concretionary in every conceivable form, now massive, now 

 cellular and now brecciated was no doubt a difficult one. No divi- 

 sions are shown in the maps. Nevertheless it is possible to arrive at 

 some fairly definite divisions in this curious formation, though we will 

 not go so far as to assert that the following scheme, propounded by the 

 late Mr. Howse, and the best known to us, can be regarded as anything 

 more than tentative. These proposed divisions are (in ascending 

 order) : 



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