Reviews — Ball's Guide to the Western Alps. 133 



granite, gneiss, and some mica-schists. Their probable metamorphic 

 origin is pointed out, but M. Desor observes that, so far as regards 

 the connexion between the orography of the Alps and their geo- 

 logical structure, it is of little imjDortance whether we consider the 

 crystalline centres as originating in the eruption of igneous rocks, 

 or in the metamorphism of old sedimentary deposits. 



The intervals or " troughs " between the higher masses are, as a 

 rule, occupied by stratified rocks of softer and less-resisting nature ; 

 they consist generally of Palaeozoic or of older Secondary strata. 

 The disturbances to which these beds have been subjected prove 

 of great intensity. During elevation the crystalline rocks which 

 underlie these beds, when lifted to a sufficient height and delivered 

 from lateral pressure, seem to have expanded in the direction of 

 least resistance, and thus has been produced the fan-structure so 

 characteristic of the central region of the Alps. 



The Alpine geologist has many difficulties to contend with in the 

 repeated folding of the strata and in the actual reversion of their 

 original order of superposition ; he has, moveover, to deal with 

 deposits which scarcely ever retain the characters that are elsewhere 

 familiar to him — the rocks are remarkablj'' altered, and fossils are 

 extremely scarce. M. Desor remarks that it is upon the outer slopes 

 of the Alps, and at a distance from the crystalline groups, that the 

 sedimentary rocks can be studied to the best advantage. Here, he 

 adds, it is sometimes possible to observe the entire series, from the 

 Paleozoic to the Miocene deposits, following each other in their 

 natural order, and so much the better displayed as they are remote 

 from the centres of disturbance. 



The author gives a brief description of the different sedimentary 

 deposits, and we should mention that this volume is accompanied by 

 a coloured geological map of the Western Alps, on a scale of about 

 five and a half miles to the inch. 



A short space is devoted to the relations between the Geology and 

 Orography of the Alps, and, in conclusion, a sketch is given of the 

 Geological History of the Alpine Regions. 



The first distinct evidence of a land-sixrface in the Alpine region 

 occurs in the Coal period. From the Coal period to the Lias the sea 

 probably again covered nearly the entire area. There is reason to 

 believe that the latter epoch was preceded and accompanied by con- 

 siderable oscillations of level, and that thence we may date the 

 commencement of a continuous barrier of dry land, separating the 

 seas that flowed on its north and south sides. 



The Secondary period was, on the whole, marked by elevation ; 

 in the Eocene period considerable changes of level occurred ; while 

 in Miocene times the plain of Switzerland was submerged to a depth 

 which permitted the accumulation of vast masses of Conglomerate 

 and of Molasse, and in some places the sea appears to have reached 

 districts which it had not touched since the Palaeozoic epoch. The 

 final iipheaval, commencing after the deposition of the Miocene 

 strata, is that which gave to the Alpine chain its existing form. 



Mr. Ball gives copious lists of works relating to the topography 



