514 Notices of Memoirs — E. E. L. Dixon — 



are found principally in this division. It is sometimes ' cellular ', 

 due to solution of fragments out of cementing material of brecciated 

 beds. This horizon has been a zone of thrusting, the amount of 

 brecciation being determined by the relative compressive strength and 

 rigidity of the strata. A flexible limestone occurs near the top of this 

 division. 150 to 200 feet. 



Claxheugh Limestone. — Yellow, earthy, friable, and crystalline 

 limestone. Generally unbedded, in places very fossiliferous, some- 

 times brecciated (crush breccia) and highly fractured. Its upper 

 surface is very irregular. Some of the brecciated beds between 

 Frenchman's Bay and Marsden, on Tynemouth Cliff, and at Blackhall 

 Hocks are included in this division. Outcrops roughly parallel to 

 coast as a continuous band. Brachiopods and most of the other genera 

 of Permian fossils stop at this horizon in Durham. 50 feet. 



Houghton Limestone. — Regularly bedded, thinly at base, more 

 coarsely above. Top layers often highly displaced and tilted up ; in 

 one or two places it is entirely thrust out of position. Thickens 

 greatly from north to south of county, and width of outcrop increases. 

 Often full of geodes. 10 to 400 feet (?). 



(The total thickness of magnesian limestone proved by boring is 

 about 800 feet.) 



Marl Slate. — Greyish, yellowish-brownish, and blackish arenaceous 

 and argillaceous laminated limestone. I^umerous fish remains. 3 or 

 4 feet thick. 



niin band of calcareous clay a few inches thick. 



Yelloiv Sands. — An incoherent sandstone generally yellow along 

 outcrop, occasionally variegated (iron oxides and manganese dioxide). 

 In pit sections often greyish or bluish. Yery variable in thickness. 

 Top originally regular, but floor on which it rests irregular. Generally 

 false-bedded, although in places, especially near the top, it is regularly 

 bedded. Grains rounded. to 150 feet. 



II. — Limestone Unconfoeitities, and their Contemporaneoits Pipes 

 AND SwALiow-HoiES.^ By E. E. L. Dixon, B.Sc, F.G.S. 



CALCAREOUS rocks differ from other commonly occurring types 

 in being appreciably soluble in atmospheric waters, and, in 

 consequence, being eroded along underground channels where situated 

 above saturation-level. Thus it is that one of their most striking 

 physiographic characteristics is the occurrence in them of numerous 

 caves and swallow-holes, of all sizes and shapes, often containing 

 either debris of overlying rocks or deposits formed in them in situ. 

 It is the purpose of this note to draw attention to the way in which 

 this characteristic is reflected in the nature of certain unconformable 

 junctions of limestones with younger rocks. 



Unconformities may be divided, for our purpose, into two groups. 

 In the first the underlying rocks have an approximately plane upper 



1 Abstract of Paper read at British Association Meeting, Winnipeg, in Section (C) 

 Geology, August, 1909. 



