224 Alfred Brammall — The Genesis of Chiastolite. 



lowering along the base of the Alps as the latest effect of the raising 

 of that chain in Tertiary times. 1 



The gradual lowering of the lake floors must have begun after the 

 maximum glaciation, for in the Zurich Valley the older moraine was 

 lowered with it, being now buried deeply below the present valley 

 floor and overlain by the younger Interglacial and post-Glacial gravel 

 beds. Thus the flexure must have continued throughout the last 

 glaciation and reached its maximum of syncline at the time of the 

 retreat of the glaciers, that is, towards the end of the Ice Age. 2 



As regards direct evidence of the Molasse flexure, my own investi- 

 gations of the subsidence of the whole area between the Lakes of 

 Zurich and Zuy have been described in previous papers, as have also 

 the phenomena along the northern slopes of the Lake of Geneva. 3 

 These investigations fully confirmed Renevier's conclusions as to 

 the flexure, which is indeed patent to anyone familiar with the 

 general reverse dip of the Molasse banks, cliffs, terraces, and knolls, 

 often conformably overlain by moraine and stratified gravel, between 

 Lausanne, Vevey, and Clarens. 4 In the basins of Thun and Lucerne 

 the conditions of the Glacial and Interglacial deposits are, owing to 

 their closer proximity to the Alps, less clearly defined and somewhat 

 more complicated than in the other basins. But here, too, there is 

 abundant evidence of the bending process, and the more the subject 

 is studied the more will it be realized that it is to the zonal Molasse 

 flexure with its syncline in the deepest and central portions of the five 

 lake basins that the lakes themselves chiefly owe their existence and 

 their preservation. 



VI. — The Genesis of Chiastolite; and its suspected Occurrence 

 in Association with a Basic Intrusive. 5 



By Alfred Brammall, B.Sc. (Lond.), F.G.S. 



(PLATE VIII.) 



Introduction. — The following account of the alteration produced in 



shale by the sill-like intrusion at Marston Jabet, near Nuneaton, 



supplements the more general paper dealing with the intrusion as 



a whole. (See Geol. Mag., April, 1915, pp. 162-8, Plate VI.) 



1 Benevier, L'Axe Anticlinal de la Molasse, 1902 ; Heim, Beitrage, xxv, 

 p. 475, 1891. 



2 The retreat of the glaciers must have been of very long duration, for in the 

 Zurich sub-Alpine valley alone there are no less than four successive moraine 

 bars across it, each of which is evidence of a long stage in the recession. 



3 A similar instance of zonal subsidence appears to be that of the Cleveland 

 (Yorks)and the Black Combe (South- West Cumberland) "ancient glacier lake" 

 districts, both of which, according to P. F. Kendall and B. Smith, were in pre- 

 Glacial times at higher levels than at present (Q.J.G.S., vol. lviii, p. 471, 1902 ; 

 vol. lxviii, p. 402, 1912). 



4 In 1906 an instructive section was exposed at Clarens at a junction of 

 roads about 60 metres above the lake, showing about 20 metres of moraine 

 with overlying stratified gravel dipping reversely like the rock strata in the 

 immediate vicinity. 



6 A supplement to the author's paper on " The Intrusive Bock of Marston 

 Jabet, Nuneaton, Warwickshire", which appeared in the April Number, 

 pp. 152-8, Plate VI, 1915. 



