Notes on the Geology of GoThridge. By G. A. Lebour. 125 



3, and 4 on the other hand are not known to the south 

 west of Corbridge, nor do they appear to be known in the 

 northernmost extension of this part of the Series. They are, in 

 fact, local beds of more or less lenticular shape, indicating areas 

 of comparatively deep sea, probably occupying arms of the sea or 

 straits some twenty or thirty miles in width, and separating 

 stretches of low-lying land (probably islands) of which the traces 

 are preserved to us in some of the thin seams of coal — seldom 

 thick enough to be worked but none the less interesting to geo- 

 logists — that occur about this horizon. What the exact area of 

 these successive straits (successive in time only) may have been 

 we have no evidence to tell, but that between Tyne and Wans- 

 beck (or perhaps Coquet) in the time which elapsed between the 

 deposition of the Little Limestone and Felltop there was, at three 

 distinct periods and occupying identical portions of what is now 

 Northumberland, a coral sea of no great width is abundantly 

 proved. That this narrow sea ran east and west, and that its 

 deepest portion was somewhere between Belsay and Stamford- 

 ham is all but certain. 



These three intercalated beds of Limestone are thus of great 

 interest, and can nowhere be better examined than in the neigh- 

 bourhood of Corbridge. All the lime quarries between the 

 Eoman Wall and the Tyne and east of the town, (except those at 

 Halton Shields and between Harlow Hill and the Southern 

 Whittledean Eeservoir) are opened in one or other of these beds, 

 in which and in the shales accompanying which, it may be men- 

 tioned in passing, most of the common fossils of the Yoredale 

 Eocks may be found. 



Coinciding with the intercalation of these Limestones is a very 

 important thickening of the whole set of beds between the Fell- 

 top and the Little Limestone, a thickening accompanied or chiefly 

 caused by a great developement of the sandy and gritty members 

 of the Series rather than by an increase in the thickness of the 

 shales. Thus, as I have elsewhere shown,* whereas the entire 

 thickness of the strata between the FeUtop and Little Lime- 

 stones is only 350 feet (with no limestones) in the Alston district, 

 it is 1450 feet in the country between Corbridge and Belsay 

 (with three thick limestones), and whereas in the former region 

 the total thickness of the sandstones is nearly equal to that of the 

 shales, the shales in the latter form scarcely one fourth of the 



* Transactions Mining Institute, vol. xxiy, p. 73 (1875). 



