314 THE STEATIGKAPHT AND TECTONICS [Aug. 1911, 



Prof. E. J. Gaewood congratulated the Author on the care and 

 perseverance of which he had given proof in working out the 

 Permian geology of an area with which the speaker had been 

 familiar from his childhocd. He was glad that the Author had 

 confirmed his own views regarding the original character of the 

 magnesium carbonate present in these beds. He asked whether the 

 Author had been able to determine the general direction of the 

 thrusting over the area to the south of the Tyne Valley, and 

 whether the movement which produced it could be shown to be con- 

 nected with the thrust described by Prof. Lebour & Dr. Smythe in 

 the Coal Measures farther north ; whether any approximate date 

 could be assigned to these movements ; and whether they belonged to 

 more than one period. The speaker referred to the difficulties in 

 explaining the V-shaped breccia-gashes, and asked the Author 

 whether he accounted for the uninterrupted character of the over- 

 lying bedding-planes on the view that these had been thrust over the 

 gashes as a whole ; were the nether surfaces of these overlying beds 

 striated in the manner which the Author had described elsewhere ? 

 He agreed with the Author that the marly magnesian matrix had 

 been removed from the beds in which the cellular structure was 

 developed, but thought that this may often have taken place by 

 the mechanical action of water. For this reason he deprecated the 

 use of the term ' dedolomitization ' for that process, as the term had 

 now come into general use for cases where the magnesium was not 

 removed, but had entered into fresh combinations. 



Mr. E. E. L. Dixon had listened with much interest to the 

 Author's discussion of several points of general importance. He 

 inquired what precisely was the agency that the Author supposed 

 to have deprived a 'dedolomitized' limestone of its magnesium. 

 Under conditions of ordinary weathering a dolomite is undoubtedly 

 more resistant than a limestone, unless, as in the case of some of 

 the dolomites in the Carboniferous Limestone of South "Wales, 

 the dissolution of a slight amount of interstitial calcite releases a 

 considerable quantity of dolomite in the form of loose crystals or 

 grains, and enables them to be washed away mechanically. The 

 disappearance of everything, including insoluble impurities, from 

 the cavities in the ' dedolomitized ' limestones would seem to 

 demand some such action. Another point raised was the origin 

 of the dolomite. What was the evidence that any of it was 

 an original, chemical precipitate ? 



The speaker was anxious to hear whether the Author considered 

 that all the masses of breccia in Marsden Bay had originated in earth- 

 movements. Prof. Lebour's explanation — that some of them, the 

 ' breccia-gashes,' had been caused by collapse of material into 

 cavities formed by solution in the limestone — had led the speaker 

 to apply the term ' gash-breccia ' to large masses of breccia, up to 

 several hundred yards across apparently, in the Carboniferous 

 Limestone of South Wales. These had undoubtedly originated by 

 collapse into solution-cavities in the heart of the pre-Triassic 

 Armorican mountain-chain ; and it would be decidedly awkward 



