520 Geological Society. 



from the base of the Alps) to Lyons, and was formed by the waters- 

 of the retreating Rhone and Arve glaciers on a Molasse-and-marl 

 plateau, the altitude of which above sea-level was 800 metres near 

 Lausanne, and 300 metres near Lyons. 



Erom this concurrent evidence in Northern Switzerland and in 

 the Rhone Valley, the author is led to conclude : 



(1) That at the time of the deposition of those alluvial cones, the 

 principal Subalpine valleys and lake-basins could not as yet have 

 existed in their present form or depth, and must have been from 

 100 to 200 and 400 metres higher ; and 



(2) That the Subalpine valleys were eroded to their present depth 

 in the course of the inter-Glacial Period — now recognized to have 

 been of very long duration — between the Pliocene and the Middle 

 Pleistocene (or maximum) glaciations, and that the Subalpine lake- 

 basins were formed in the same period by the contemporaneous 

 action of fluviatile erosion and of a zonal settling along the base 

 of the Alps after these had been raised by horizontal pressure. 



2. ' Overthrusts and other Disturbances in the Braysdown Col- 

 liery (Somerset), and the Bearing of these Phenomena upon the 

 Effects of Overthrust-Eaults in the Somerset Coalfield in general/ 

 By Frederick Anthony Steart, Esq. 



This coalfield, although covered by comparativel}' undisturbed 

 Secondary rocks, is in part the most disturbed and contorted of 

 those known and worked in the United Kingdom. It is seldom, in 

 some parts of it, that one sees 200 yards of coal without a fault 

 or other disturbance. The ' Radstock Seams ' of the Upper Coal- 

 Measures at Radstock are traversed by a huge ' overlap- fault, 

 which thrusts them forward for a great distance ; this runs nearly 

 east and west, and has parallel to it two smaller overthrusts. In 

 one of them the coal at first dips towards the thrust, then it 

 thickens from 2 to 6 or 8 feet, next it becomes inverted, and 

 eventually regains its former character. The continuity of the coal 

 has been proved in the case of three of the coal-veins. As there is 

 practically the same sequence of strata on both sides of the fault, 

 it is concluded that the ' overthrusts ' did not take place till all the 

 coal-seams of the Radstock Series had been deposited. The areas of 

 ' dead ground/ sometimes considered to be wash-outs, are probably 

 also the result of movement. The areas occur near faults, frequently 

 take a course parallel to overthrust-faults, and, at their margins, 

 the coals are often reduplicated. ' Dead ground ' is usually found 

 only in those seams which lie on a floor of soft black shale, and some- 

 times, instead of dead ground, there are large areas with very thin 

 coal. The flat roofs of coal-seams in the dead ground are invariably 

 striated. The authors theory is that all these effects have been 

 caused by the gradually increasing movement of the strata from the 

 top seam or ' Great Vein ; to the bottom seam or ' Bull Yein.' The 

 coal-seams nearly always thin from their undersides upward, as 

 though the floor had moved farther, or at a greater rate, than 

 the roof. 



