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289 



THURSDAY, JULY 28, 1887. 



THE GEOLOGY OF NORTHUMBERLAND AND 

 DURHAM. 



Outlines of the Geology of Northumberland and Dur- 

 ham. By Prof. G. A. Lebour. (Newcastle-upon-Tyne : 

 Lambert and Co., 1886.) 



THE normal guide-book cannot be said to be as a rule 

 very entertaining reading, and a work like the one 

 before us is essentially a geological guide-book. But the 

 guide-book may be so treated as to present points of 

 interest even to the reader who never puts it to the use 

 for which it was primarily intended. Such a guide-book 

 has been produced by the joint labours of a great poet 

 and a great geologist ; and a great historian, when he 

 leads us round towns and cities thick with objects full of 

 historical associations, puts into our hands a guide-book 

 of this type. No ooe, least of all the author, would for a 

 moment think of placing the unpretending little volume 

 on the geology of Northumberland and Durham in the 

 same class as the books to which allusion has just been 

 made, but it is very curious to note how many questions 

 of interest are started during the perusal of what at first 

 sight looks like nothing more than a rather dry descrip- 

 tion of local geology. Some of these points may now be 

 noticed. 



The fact that peat-lakes have often more than one 

 outlet, though it is coupled by the rather questionable 

 statement that " ordinary lakes with two outlets do not 

 exist," throws light on a much disputed question in 

 physical geography. For two outlets to co -exist for any 

 length of time in a lake, it seems necessary that the 

 outflowing streams should have the same eroding power, 

 and this will be the case if these streams have the same 

 fall, and if their beds are composed of the same material. 

 A lake in Arran which has two outlets is wholly sur- 

 rounded by granite, and in its case the two conditions 

 mentioned are probably satisfied. The eroding power of 

 the sluggish outflows from a bog must be very small, and 

 the material on which it is exerted is everywhere peat. 

 Here it is easy to realize the possibility of there being 

 several outlets. But there are few cases in which the 

 balance of power could be exactly maintained, and hence 

 lakes with two outlets must be rare. 



It is extremely interesting to find a Lingula recorded 

 from beds high up in the Coal-measures. New cases of 

 marine bands in the upper portion of the Coal-measures 

 are constantly being brought to light, and each fresh 

 discovery strengthens the belief that the occasional 

 presence of marine forms is not confined to the 

 Lower Coal-measures or Canister Beds, or is even com- 

 moner there than in the Middle Coal-measures. If this 

 be so, the attempts which have been made to draw on 

 palaeontological grounds a line between the Lower and 

 Middle Coal-measures, at once fall to the ground. 

 Marine bands seem to be less plentiful in the Coal- 

 measures of Durham and Northumberland than in those 

 of Lancashire and Yorkshire. This accords well with 

 the hypothesis that the outlets which connected the Coal- 

 measure lake or estuary with the open ocean lay to the 

 Vol. XXXVI. — No. 926. 



west. It was through these openings that marine forms 

 now and again migrated into the area, and the further a 

 spot was from the door of entry, the fewer would be the 

 immigrants which reached it. 



Under the head of "Millstone Grit" we are told that 

 the rocks, which in Lancashire and Yorkshire are con- 

 spicuous under this name, are in Northumberland in no 

 wise distinguished from the Coal-measures proper, that 

 they have no distinctive fossils — in short, nothing peculiar 

 to them but their position. But the contrast between 

 Lancashire and Yorkshire on the one hand, and North- 

 umberland on the other, is really by no means so great 

 as these words would seem to imply. It is true that in 

 the first-named and adjacent counties a portion of the 

 Carboniferous rocks has certain lithological peculiarities 

 so strongly marked, that it is convenient to separate it 

 from the beds above and call it Millstone Grit, but the 

 distinction rests solely on the comparatively unimportant 

 points of coarseness of grain and massiveness, and when 

 we look to points of real importance, such as conditions 

 of deposition, fossils, and the like. Millstone Grit, Canister 

 Beds, Coal-measures, and other similar groups are seen 

 to be arbitrary, though very convenient, subdivisions of 

 a formation that is essentially one from top to bottom. 



Prof. Lebour has happily seized on the only line of 

 demarcation among the Carboniferous rocks which can 

 have any real significance ; that, namely, which separates 

 rocks in which the fossils are all practically marine from 

 rocks in which marine fossils are the exception, and in 

 which they are the exuvicn of marine creatures which 

 paid occasional visits to the area, but whose stay there 

 was short. 



And this brings us to the lower and marine portion of the 

 Carboniferous system, which is divided in the present work 

 into two members, named respectively the Bar nician and the 

 Tuedian. The contrast between the Mountain Limestone 

 of Derbyshire and Yorkshire, almost pure limestone from 

 top to bottom, and the beds in the south of Scotland, 

 which we must look upon as its time-equivalents — shales 

 and limestones in which it is often difficult to find lime- 

 stones at all, and more difficult still to recognize them 

 when they are come across — this contrast has become one 

 of the hackneyed instances of geology. The name Ber- 

 nician is applied by Prof. Lebour to beds on the same 

 geological horizon in the north of England. They are in 

 a general way intermediate in character between their 

 equivalents on the north and on the south ; but in a work 

 intended mainly for the use of young students the author 

 has wisely warned his readers that they will not find in 

 Nature that regularity and uniformity of change that some 

 geological diagrams might lead them to expect. The lime- 

 stones are not all wedges with their sharp edges pointing 

 north, and, moreover, the total thickness both of the whole 

 group and of its various subdivisions varies very much 

 from place to place. We would suggest, in the interest 

 of those students who have not yet got beyond books, 

 that it be pointed out in the next edition that this is only 

 what was to be expected ; that in a subsiding area it is 

 almost certain that sinking will go on faster at some spots 

 than at others ; that pits and holes will be thus formed in 

 the sea-bed ; and that in a deposit laid down under such 

 conditions great variations in thickness and character 

 must necessarily arise. It would not be amiss to call 



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