160 SCIENCE PROGRESS. 



lustres" of Mount Jo vet must be either Triassic or post- 

 Triassic. The mountain, however, has also been described 

 by Zaccagna (7), according to whom the structure of the 

 mountain is totally different. He agrees that the summit 

 is of " schistes lustres," and the lower slopes of Trias. 

 But he says that the former is a great central mass, and 

 that the Trias was deposited against its flanks. He is as 

 positive that the "schistes lustres" are older than the 

 Trias as M. Bertrand is of the reverse. 



These two mountains therefore supply a crucial test, 

 and it is to be hoped that they will be redescribed by some 

 independent geologist. For while these two authors are in 

 such absolute contradiction as to the fundamental facts of the 

 question, it is impossible for any one to form an opinion 

 without an actual examination of the country. 



In addition to his interpretation of the structure of these 

 mountains, M. Bertrand rests his case on the evidence of the 

 general structure of the region. He regards the Alpine chain 

 in this part of its course as formed of a great arch, of which 

 the sides have been so closely driven inwards that the sum- 

 mit of the arch became wider than the base, and the two 

 limbs were inverted. The arch is therefore regarded as a 

 typical Alpine fan. The central line of the Alps Bertrand 

 thinks is occupied by a band of Carboniferous deposits, 

 which lie along the axis of this fan-shaped fold. The beds 

 beside it are inverted, but those at a little distance are 

 inclined outwards, and occur in their normal sequence. 

 This " fan structure " was at one time continually invoked 

 in order to explain away awkward facts in Alpine geology. 

 But recent work has overthrown some of the most classical 

 cases of this phenomenon, and one feels very inclined to 

 look suspiciously on any attempt to resuscitate it. M. 

 Bertrand makes one variation in the theory, which helps 

 him to apply it, but may not help others to accept it. He 

 regards the central gneisses and some of the other massifs 

 of the area as amygdaloid lenticles or nuclei, wedged in 

 between the main folds of the district. Thus, e.g., in the 

 case of Mont Blanc, he represents it as an anticlinal 

 nucleus cropping out on the axis of a synclinal fold. This 



