408 SCIENCE PROGRESS. 



often in their condition very similar to the crystalline, not 

 only skirt the Centralmassif, but often penetrate it, and 

 form an integral portion of its construction. These are not 

 separated portions embedded, but the remains of closely 

 compressed troughs. The Centralmassifs have not folded 

 the sedimentary chains by their lateral activity, but are 

 themselves zones of the earth's crust which were formerly 

 overlaid by sediments, have since undergone compression, 

 and are now exposed through denudation and erosion. The 

 folding of the Centralmassif is of younger tertiary age, and 

 therefore synchronous with that of the sedimentaries ; any 

 older flexuring could, at most, have been of a very feeble 

 character {loc. cit., p. 178). All the Centralmassifs and all 

 the limestone chains of the Alps have been formed, because 

 the sum total of their foldings represents the force necessary 

 to neutralise a definite tangential compression. The innu- 

 merable magnificent ridges and peaks, so varied in form 

 and outline, are the outcome of the same activity and of 

 the same period, no matter whatever may be their materials; 

 they are probably the result of a pressure able to overcome 

 the cohesion of the rocks and producing folds, whereas the 

 remainder of the earth's crust had not shrunk one-hundredth 

 part towards its centre {loc. cit., p. 186). 



Having thus established the principle that folding is the 

 main and necessary result of the compression, he proceeds 

 to a study of the folds themselves, these being either 

 normal, with the beds dipping away from the axis plane in 

 the arch, and towards it in the trough ; isoclinal, where both 

 the anticlinals and synclinals are in such a position that the 

 beds are parallel to the axis planes and appear to be con- 

 cordant in stratification ; and fan-shaped when the strata 

 both in the anticlines and synclines dip towards the axis 

 planes, producing by this means the appearance of the 

 younger rocks underlying the older. Should this latter 

 form become very oblique, either a lying isoclinal overfold 

 or fan-shaped one may be produced, the middle limb of 

 which may be squeezed out. These may pass directly into 

 fold faults, there being no strict boundary between the latter 

 and the overthrust planes. 



