TIIK AUCH^AN ROCKS 61 



purple conglomerate which forms the base of the Cambrian System, 

 am I no fossils have been found in them except some microscopic 

 bodies which resemble Kadiolaria and Foraminifera. 



The Central Plateau. This is a large region extending from 

 La Chatre and Moulins on the north (Departments of the Indre 

 and Allier) to the Cevennes in the south, a distance of about 

 220 miles, while, between Chalus and Brive on the west to the 

 valley of the Rhone on the east, its breadth is over 160 miles. 



This great area consists of a complex of granites, granulites, 

 gneisses of several kinds, micaceous schists, and phyllites, but to 

 what extent these rocks are of pre-Cambrian age is at present 

 uncertain, because the Palaeozoic rocks by which they are doubtless 

 surrounded are almost everywhere concealed and overstepped by 

 Mesozoic strata. It is only in the southern border (in the Tarn 

 and Herault Departments) that rocks of Palaeozoic age are found in 

 contact with them. 



The granites are doubtless all intrusive rocks and the granitoid 

 gneisses may be foliated portions of the same intrusions, like those of 

 Donegal ; while the more highly crystalline portions of the complex 

 are probably those which are most closely associated with these 

 intrusions, being merely the more altered portions of the mica- 

 schist series into which the igneous rocks were intruded. No break 

 or clear plane of separation has been detected in any part of the 

 schistose series, but in several districts along or near the borders 

 of the region there are tracts of still less altered and obviously 

 sedimentary rocks, including arkose sandstones, quartzites, slates, 

 and phyllites comparable with the Brioverian. A long strip of 

 such rocks occurs between Brive and Tulle in the Correre, where 

 Messrs, de Launay and Mouret have described the following 

 descending succession : 30 



Dark-grey slates and argiilites. 



Green slates and phyllites. 



Greenish sericitic and micaceous quartzites. 



Similarly in the Cevennes on the outer borders of the gneiasic 

 rocks there is a considerable thickness of satiny schists or phyllitw 

 (schistes luisantes). In the district of Montagne Noir these schisto 

 include bands of limestone and appear to pa*s up into a set of 

 shales and limestones which contain Cambrian fossils. H?nce 1C. 

 Bergeron regards the whole mass of the Montagne Noir a* 

 Cambrian, 81 repeated in many parallel flexures and becoming more 

 and more metamorphosed toward the central axis of the mass 

 till the rocks pass into mica-schists and gneisses. He states that 

 the limestones resist alteration longer than the shales, but gradually 



