C— GEOLOGY. 78 



the Theux outcrops is manifestly a dislocation, and early workers 

 explained the local occurrence of the relatively late formations (Middle 

 Devonian to Carboniferous) as due to preservation within an incomplete 

 cauldron-subsidence. Fourmarier, however, by careful comparison of 

 facies showed that the rocks of the surrounding country have travelled 

 northwards relatively to those of the Theux exposure. To account for 

 this horizontal displacement he necessarily interpreted the boundary 

 dislocation at Theux as a low-angled thrust, cut through by erosion. He 

 also identified the newly recognised thrust with the Eifel Thrust, well 

 known in the country to the north. Before long Fourmarier's views were 

 dramatically established by boring. The Carboniferous outcrop at 

 Theux is separated by Devonian hills, three miles wide, from the exploited 

 coalfield to the north. This separation has been proved to be merely 

 superficial. Two deep bores, put down on Fourmarier's advice, pierced 

 the Devonian and penetrated far into underlying Carboniferous. No coal 

 seam was discovered, but the result was very justly hailed as a signal 

 triumph for geology. 



The preparatory hinged subsidence that we have met with in the history 

 of the Caledonian Chain, in southern Scotland and again in Canada, re- 

 appears in the Hercynian record of western Europe. Broadly speaking, 

 the Devonian of the Hercynian Foreland is continental (Old Red Sand- 

 stone), while that of the Hercynian Mountains is marine. Two main 

 regions can be distinguished in the foreland, an eastern and a western. 

 In the eastern. Lower Devonian is generally absent, while Middle and 

 Upper Devonian are locally developed — in Belgium and the Baltic, but not 

 in Orcadia, the upper division of the Middle Devonian is frankly marine. 

 In the western region of the foreland, which includes England, Ireland and 

 the south and west of Scotland, Lower and Upper Devonian are widely 

 represented, in both cases as Old Red Sandstone, while Middle Devonian is 

 unknown. The Devonian of the mountain land is fairly complete and 

 predominantly marine, both in the east and the west ; and it seems to 

 have derived much detrital material from the north. Evidently this 

 marine Devonian gathered on a tectonic slope that, descending south- 

 wards to the site of the future mountains, was constantly renewed by 

 subsidence. The contrast between the foreland and the mountain region 

 is particularly striking along the Franco-Belgian front of the chain. It 

 has been exaggerated, as is so often the case, by overthrusting of regions 

 previously separate ; but even so the pre-thrusting contrast must have 

 been thoroughly noteworthy. The Lower Devonian and the lower part 

 of the Middle Devonian of the thrust region sometimes total 17,000 feet, 

 while both divisions are absent in the over-ridden foreland to the north. 

 The line at which this great mass of sediment fails is known as the Condroz 

 Crest, and was familiar to Cornet and Briart when they wrote their cl ssic 

 paper of 1877. To-day its course has been followed for 200 miles along 

 the strike. I prefer to speak of it, when concerned with its prc-thrust 

 character, as the Condroz Slope. 



During Lower Carboniferous times, marine transgression submerged 

 the Hercynian Foreland far and wide. A northern continent persisted, 

 but its waste was retained along a deltaic belt that stretched through 

 southern Scotland and northern Ireland. Accordingly, clear shallow 



