GEOLOGY 



asserted that had the Millstone Grit not been known and mapped in the 

 more southern counties, its representatives in Durham (and still less in 

 Northumberland) would probably not have been recognized as forming 

 a separate stratigraphical group. They would no doubt have been re- 

 garded simply as a set of rather coarse, irregular and variable gritty sand- 

 stones, with some shales and one or two thin local coal-seams, forming the 

 basal portion of the Coal Measures : as the introduction in fact to the huge 

 non-marine set of strata to which the term Coal Measures is properly 

 applied. However, as the division is generally recognized it is best to 

 retain it, bearing in mind the want of special features which is its only 

 noticeable, if negative, character. In Durham these beds, though no- 

 where more than 400 or 500 feet thick, and often much thinner, by 

 reason of the orographical features of the country occupy a considerable 

 area. The hills covered with heathery moorland, which rise between 

 the deep dales dug out of the Bernician rocks, are capped with this de- 

 based Millstone Grit, and much of the wild crag, ling and peat scenery 

 on these high grounds is due to the unyielding nature of these silicious 

 deposits. It should be stated however that in most of the geological 

 maps of this part of England published before the sheets of the Geolo- 

 gical Survey the area coloured as Millstone Grit is very much exaggerated, 

 partly owing to a real misconception as to the distribution of the strata, 

 but partly also to the fact that the older geologists were in the habit of 

 grouping a good deal of the Bernician Series (even including the Great 

 Limestone in some cases) under the appellation Millstone Grit. 



After what has been said above it will be readily understood that 

 between the Millstone Grit and the overlying Coal Measures no violent 

 break is to be expected in this county. Not only is this the case, but it 

 can be truly said that none but a purely arbitrary and non-natural 

 boundary can be drawn between the two. One can go still further than 

 this and state that even such an arbitrary line of demarcation can scarcely 

 be drawn with any confidence. Thus it has repeatedly happened that 

 the writer has been called in by coal owners to decide whether in the 

 bore holes which they had put down below the known workable coal 

 seams of the Coal Measures the Millstone Grit had been reached or not, 

 and he has been quite unable to give more than a tentative and generally 

 a very doubtful opinion. There is in fact nothing but a perfect passage 

 between the two, a passage unmarked by any datum line recognizable 

 over any but the most limited areas. This difficulty is intensified by the 

 entirely artificial divisions which, for mere convenience, have been usually 

 accepted in classifying the Coal Measures. These divisions are, as re- 

 gards the upper two, taken as including certain well-known coal seams, 

 and for the practical purposes of the miner this is no doubt a useful 

 arrangement. But the lowest division — known as the Lower Coal Mea- 

 sures or Gannister Series — though sufficiently limited at the top by this 

 method of classification, lacks any similar means of fixing its bottom 

 limit, as there are thereabouts no coal seams at all. 



The Lower Coal Measures then (which must in no wise be con- 



II 



