part 2] OF THE SOUTH WALES COALf lELI). 163 



little further tilting ; while at a later date that along the north-east 

 side of the basin received a tilt agreeing in amount with the first. 

 Thus the same continuous limestone-band between Mitcheldean and 

 the Blackpool Valley, dipping almost uniformly at from 60° to 70°, 

 must be supposed to have been tilted to the same extent at two 

 different periods : one part before, and another after, the deposition 

 of the Coal Measures. The speaker maintained that the field 

 evidence, part of which was indicated, was decisively in favour of 

 the view that dip-faults and strike-faults together had produced 

 the appearances attributed to overstep, and thought that the un- 

 conformity' at the base of the Coal Measures was probably so gentle 

 that the dip of these, as seen in single exposures, might not be 

 visibl}' different from that of the underlying stratum. He de- 

 murred, therefore, to the term ' flagrant unconformity ' employed 

 in the abstract of the paper. 



Mr. F. DiXEY, in referring to the very irregular surface of the 

 Carboniferous Limestone upon which the Millstone Grrit rests near 

 Abergavenny, and to pipes and caverns of Millstone G-rit in the 

 Carboniferous Limestone, mentioned that all the above features, 

 described by Dr. Strahan and Mr. Dixon as occurring on the 

 eastern margin of the South Wales Coalfield, are reproduced in 

 detail near Hav^erfordwest, on the southern margin of the Pem- 

 brokeshire Coalfield, where the Millstone Grit rests unconformably 

 upon the Dihunophyllum beds of the Carboniferous Limestone. 



Prof. E. J. GrARWOOD congratulated the Authors on the com- 

 pletion of an important piece of work showing the value of 

 detailed zoning in the Lower Carboniferous rocks. He commented 

 on the numerous facts of interest which the paper contained. He 

 would have liked to ask for further information about some of 

 the points brought forward ; but, owing to the lateness of the 

 hour, he would confine his observations to the question of the two 

 periods of dolomitization described by the Authors. In West- 

 morland the dolomitization of the Lower Limestones in the Shap 

 district could be proved to have taken place contemporaneously 

 with the deposition of the beds ; but the rocks also contained 

 cavities lined with crystals of dolomite, which must have been 

 deposited subsequently to the consolidation of the deposit. In 

 this case the material for the crystals must have been leached out 

 of the upper portion of the dolomite and redeposited from solution : 

 the speaker would like to ask the Authors whether the secondary 

 dolomitization described by them might not be due to a similar 

 process, or whether they had evidence that the material which 

 produced the second dolomitization had been introduced at a 

 subsequent geological period. 



Prof. T. F. SiBLT thanked Dr. Strahan, Mr. Dixon, and 

 Prof. G-arwood for their appreciative remarks. Mr. Dixon's inter- 

 esting description of the phenomena on the eastern and northern 

 margins of the coalfield afforded a gratifying confirmation of the 

 Authors' conclusions. As to the problem of dolomitization dis- 

 cussed by Prof. Garwood, the vein-dolomites were developed on too 



Q. J. G. S. No. 290. o 



