STEVENSON, GOAL BASIN OF DECAZEVILLE, FRANCE 245 



the materials, the directions of the contributing streams, the development 

 and the interferences of the little deltas during advance from the shore. 

 This admirable synopsis has been utilized throughout b}' the writer; its 

 clear descriptions enabled him to pass rapidly over some portions, ren- 

 dered unnecessary the examination of others and made possible the practi- 

 cal confining of his attention to localities in which the phenomena of coal 

 accumulation are shown most notably.^ 



ROCKS SUREOUIS^DIIS^G THE BASIN 



Long ago, it was recognized that the basin of Decazeville, like those on 

 the central plateau, occupied a depression in Archean rocks ; the mica 

 schists with granite being continuous from the Commentry to the Decaze- 

 ville basin. But patches of other rocks exist, whose distribution on the 

 borders is of great importance to one endeavoring to trace the history of 

 the area. Bergeron in his first paper gave some notes respecting these 

 rocks, but the later memoir by Bergeron, Jardel and Picandet gives in 

 detail their distribution along the approximately 30 miles of contour. A 

 synopsis of their statement is essential, though the relations are shown on 

 the accompanying map,* page 246. 



The prevailing rock is mica schist, containing here and there some 

 granulite and occasionally becoming gneissoid. Granite appears at the 

 southwest, and, on the western side, a narrow strip of gneiss extends north- 

 ward from the southern boundary for about four miles. Just beyond is 

 a mass of granite, sharply defined as if by a fault at the west, which is 

 about half a mil^ wide and three miles long, extending almost to the Eiou 

 Mort and showing on its eastern side a very narrow strip of mica schist. 

 A small area of microgranulite is cut by the Eiou Mort. At a little way 

 north from that stream, one reaches andesite which extends to the north- 

 ern end of the basin, forming the western boundary for about four miles. 



Andesite forms the eastern boundary also to beyond the Lot Eiver. It 

 is almost continuous across the basin at the extreme north, but it has 

 suffered great and irregular erosion in the broad valley of the Lot, and 

 the Coal Measures reach eastward to the mica schists. These form the 

 boundary for somewhat more than four miles, to be succeeded by grani- 



3 I must express my high appreciation of courtesies received from M. H. Fayol, Direc- 

 tor of the Comp. Anon, de Commentry, etc., M. A. Jardel, Director of that company's 

 mines in the Decazeville hasin, and M. E. Brocard, the metallurgical engineer of the 

 company, as well as from M. Saint-Martin, geometrician of the Campagnac company. 

 The generous assistance given by these officials makes me regret keenly that the studies 

 at Decazeville compel me to the belief that the coals of that basin, like those of Com- 

 mentry, are of in situ origin. 



*' This map has been taken from the memoir by Bergeron, Jardel and Picandet ; the 

 scale has been reduced slightly, but no change has been made in the geology. 



