364 SECTIONAL TRANSACTIONS.— C. 



7. Prof. A. GiLLiGAN. — Sandstone Dykes or Rock-Riders in the 



Cumberland Coalfield. 



Those more particularly described were met witli in the workings of the 

 Bannock Band and Main Band seams at Ladysmith Pit, Whitehaven. The 

 dykes certainly pass through the Bannock Band and Main Band seams and 

 the intervening measures, which are about 54 feet thick ; but their full vertical 

 extent has not been determined. Their horizontal extent is variable ; the longest 

 has been traced for more than a mile. They all run practically parallel to one 

 another in a direction N.N.W. and S.S.E., which is the direction of the main 

 system of faults affecting the associated measures. The average width of the 

 dykes is from two to four inches and the coal in sharp contact with them is 

 unaffected. 



The dykes do not pass up into the overlying Whitehaven sandstone. 



The author argues for the pene-contemporaneous formation of these dykes 

 and the associated measures. 



Monday, September 11. 



8. Discussion on Wegener's Hypothesis of Continental Drift. 



(a) Opener: Dr. J. W. Evans, F.E.S. 



The occurrence of allied forms of life on continents separated by great 

 oceans has given rise to speculations as to former connections between them. 

 There is in some cases also considerable similarity in the geological features 

 on the opposite sides of the oceans, especially in the case of the Palaeozoic rocks 

 of the two sides of the Atlantic and of the Pefmo-Carboniferous formations of 

 India, Australia, South Africa, the Falklands and South America. It was, 

 however, primarily to solve the distribution of life forms that. Dr. Wegener 

 formulated his hypotliesis of the dispersion of *he continental masses. He 

 supposes these to have been aggregated together as late as the Tertiary period, 

 and North America to have been in close proximity to Europe even in the 

 Pleistocene. The continents are, he thinks, slowly drifting from the poles, 

 and from east to west. The former movement is believed by him to be proved 

 by observations at European observatories and at that at Washington, showing 

 a decrease in latitude, but recent observations on the Pacific coast indicate that 

 latitude is there increasing. He is under the impression that America is going 

 west faster than Europe, and that the longitude of Cambridge, ]Mass., is 

 increasing while Greenland is moving in the same direction at a still more 

 rapid rate. Tlie continents are composed largely of rocks rich in silica and 

 alumina, ' Sial,' while those below the sea are less siliceous and heavier and 

 have been referred to as ' Sima.' This extends under the Sial of the continents 

 at a depth which Wegener estimates at 57 miles, which is probably far too 

 great. He believes that the Sial masses drift through the Sima like icebergs 

 through the sea. The Sima is, however, a crystalline solid down to a depth of 

 ]5 miles, and is as strong as, if not stronger than, the Sial. 



By overthrusts and crumpling some areas are brought nearer together, and 

 by Assuring, igneous intrusions and normal faulting others are moving apart. 

 There is some evidence of a slow drift away from Africa and towards the 

 Pacific, but there is nothing to show that this has been as rapid as is assumed 

 by Dr. Wegener. The Atlantic may have come into existence since Carboni- 

 ferous times, but this would not mean a separation equal to its whole width, as 

 part of the submergence would be caused by faulting down towards a region 

 of tension. 



The Astronomer-Royal of Scotland has shown that determinations of 

 longitude by telegraphy are subject to serious errors. Light is thrown on this 

 by the observations of Hecker and others with horizontal pendulums. In 

 addition to the variations of direction of gravity due to the sun and moon, 

 the earth's crust is subject to appreciable diurnal tilting as the result c.f solar 

 radiation. Similar seasonal effects must also occur. Variations of the barometer 

 in adjoining areas and of the underground water-level, marine tides and irre- 

 gularities in refraction, are also important. Most of these effects are con- 

 siderably less at some depth below the surface, and it is suggested that the 



