GEOLOGY 



that river to Darlington and north to Seaton Carew is made up of these 

 red rocks. Much is known of them however by means of the many 

 borings which, within the last twenty years, have been put down through 

 them in search of the valuable salt beds which they contain. The age 

 of the series has been the subject of some controversy, which need be 

 referred to here but briefly. That the lowest members of the series 

 (which nevertheless differ but slightly from the rest) are of Upper Per- 

 mian age has been held by several geologists because a few thin beds of 

 Magnesian Limestone occur in them similar in all respects to the main 

 mass of that formation below. The late Sir Andrew Ramsay, Mr. R. 

 Howse and the present writer took this view and were disposed to 

 include some of the red beds above these bands of dolomitic limestone as 

 well in the Permian System, including the lowest, at least, of the beds of 

 rock salt. Others, including Mr. H. Howell and the Geological Survey, 

 regard the whole of the red series as Triassic and — since the Bunter or 

 Lower Trias has been shown by the Survey to thin out and disappear 

 some 20 miles or so to the south of the Tees — as strata of Keuper age 

 (Upper Trias) overlapping the Bunter. The absence of well marked 

 unconformities and of any paljEontological evidence must probably 

 always leave the decision of these points doubtful, and it is therefore 

 safer, in our present state of knowledge, to adopt some descriptive non- 

 committal term, such as ' the Salt Measures,' to which no reasonable 

 exception can be taken. If the unconformity which it has been hinted 

 may possibly occur at Norton and account for the abnormal thin- 

 ness of the Magnesian Limestone there, should some day be proved, 

 then the Survey view will properly prevail and all the red beds above 

 the highest of the limestone bands be classed as Keuper. 



The salt beds, one of which is from 60 to 100 feet thick, are 

 associated with many layers of gypsum and anhydrite (the latter being 

 known to the salt-borers as 'white stone'), and the mode of their occur- 

 rence is in all respects comparable to what obtains in the Triassic salt- 

 bearing series of Cheshire. They lie in the lower portion of the series, 

 and being composed of very soluble material they thin out gradually 

 before reaching the surface. Thus the further to the dip (that is to say, 

 the further away from the original outcrop) one bores for the salt the 

 more likely one is to find it and the thicker it will be. This is why the 

 bores through which the brine is extracted are all clustered close to the 

 Tees and why they are so deep. Attempts to tap the same beds where 

 this horizon approaches the surface have either failed altogether or have 

 only met with deposits so reduced in bulk as to be comparatively worth- 

 less. As is the case with most districts underlain by easily soluble 

 rocks, subsidences are not unknown in the Salt Measure area of Durham, 

 but fortunately the great depth of the salt-winnings has prevented the 

 actual workings from causing the dire effects which have followed such 

 undertakings elsewhere. The surface sinkings are here few and due 

 altogether to the natural solution and removal of salt or gypsum at no great 

 distance from the outcrop. The best known are curious depressions at 



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