OLDEE EOCKS OF BKITTASTY. 



321 



eastwards to beyond the Rhine, more than 35° of longitnde. Yet the 

 sea now flows where some of its highest summits may have risen ; its 

 only record is preserved in the low plateaux and comparatively humble 

 hills of Cornwall and Devon, of the Channel Islands, and of Brittany. 

 Millions of cubic yards of rock must have been removed by denuda- 

 tion, which must have helped to form the conglomerates, sands bones, 

 and clays of the Secondary and Tertiary deposits of Southern England 

 and North-western France. Like the Alps, this great mountain 

 massif consisted of a foundation of presumably Archaean cr) T stalline 

 rocks, which, after undergoing extensive denudation during a long 

 interval of time, was affected by a downward bending of the earth's 

 crust, and received great accumulations of sediment, obtained, no 

 doubt, by the destruction of more remote portions of the ancient land- 

 surfaces*, and then the whole, by new earth-movements, probably 

 in new directions, was again upheaved ; the fundamental crystalline 

 rocks, both igneous and " metamorphic," were folded and, in 

 places, crushed so as to assume a cleavage-foliation, while the softer 

 and newer sedimentaries were more markedly plicated, were often 

 cleaved, and in some places underwent other modifications from 

 intense pressure. But in the Alps, although there appears to have 

 been a precarboniferous movement of some importance, the great 

 downward bending did not commence till at least some time after 

 the mountain-making process had ceased in the other region. The 

 one great geo-synclinal is of Palaeozoic, the other is of Mesozoic age. 

 The one mountain-mass belongs to the close of the Palaeozoic, the 

 other hardly began before the commencement of the Cainozoic, and 

 was in process of making during the earlier half of that period. As 

 yet, sufficient materials hardly exist for the classification and corre- 

 lation of the greater earth-movements which have modified the 

 physical structure of the European region, but they are gradually 

 accumulating, and cannot fail some day to lead to most important 

 results. 



EXPLANATION OP PLATE XVII. 



Fig.- 1. Section cut from one of the more quartzo-felspathic layers in thjB con- 

 torted banded gneiss at the estuary of Le Pouldu. (X 27.) 

 The figure shows one of the elongated irregular grains of felspar con- 

 taining inclusions of quartz (left white), and with indications of 

 cleavage-planes roughly at right angles to the longer axis of the 

 grain. This lies in a ground-mass of granules of quartz and felspar 

 (chiefly), the latter sometimes occurring in elongated streaks. (See 

 p. 309.) 



Fig^ %. Section cut from one of the more micaceous layers in the above-named 

 rock, (x 27.) 



The figure shows the elongated felspar grains (as in fig. 1) broken and 

 displaced. The rock contains crystals of mica of fair size and 

 apparently original constituents, in the direction of which ttaacrcg&s 

 traversing the slide often run. 



(For the Discussion on this paper see p. 335.) 



* On this subject there is a suggestive paper by Dr. Barrois in Ann. Soc. Geol. 

 Nord, t. xi. p. 278. The central area (that?now occupied by the western part 

 of the English Channel) appears to have remained longest above water. 



