524 



SCIENCE. 



[N. S. Vol. I. No. 19. 



figures with the much higher values ob- 

 tained by Heim, who places the shortening 

 of the arc in the north and central Swiss- 

 Alps at one-half. He discredits the struc- 

 tural interpretations which have led Heim 

 to so large a value. He takes issue with 

 the latter, particularly in the interpretation 

 of the so-called Glarner double fold, and 

 discusses this structui-e at length in an ap- 

 pendix to the volume, interpreting the 

 structure as an overthrust and not a double 

 fold. 



In discussing the mechanics of the lateral 

 thrust, to which all are agreed the Alpine 

 structure is due, the author says the earth's 

 crust may be considered a virtual arch. 

 Then the continents must be either arches 

 of less radius than that of the earth as a 

 whole, or they must be superficial masses 

 reposing upon the arch. In the latter case 

 the continental masses would suffer no fold- 

 ing, but would lie as a dead weight upon 

 the laterally compressed and folding arch 

 below. This being contrary to experience, 

 it is rejected, and the alternative is adopted 

 that the continents are arches of smaller 

 radius. The condition of folding of strata 

 by lateral compression is, then, that they 

 must lie below the limiting curve of the 

 continental arch. So long as they lie above 

 this curve they escape folding. Where 

 folding occurs under the dead weight of 

 rocks lying above the curve it is manifest 

 at the surface only as elevation or depres- 

 sion. But the load tends to restrain fold- 

 ing and the latter takes place most readily 

 where the load is least. This occurs where 

 the continental arch merges into the geoid 

 arch. Here is the weakest part of the 

 arch ; here the strongest folding should 

 arise. Orogenic folding is most effective 

 on the borders of the oceans. This fact the 

 author finds in accord with his theoretical 

 deductions, for it is on the oceanic borders 

 that the continental and geoid arches inter- 

 sect. 



This principle is resorted to in explana- 

 tion of the common up-throw of sj'nclinal 

 troughs. The deep sjTicliual folds will suf^ 

 fer most from the lateral compression. The 

 consequence is that the axial troughs of the 

 syncUnes are faulted up and the anticlines 

 relatively dropped. 



Part of the transverse cleavage of the 

 rocks is ascribable to pre-Permian oro- 

 genic forces and part to the later com- 

 pression which gave rise to the Alps. 

 Most of the pre-Permian strata show 

 this cleavage in a pronounced degree. 

 This cleavage is best developed in the Zil- 

 lerthaler towards the middle of the section, 

 and least so on the margins of the Alpine 

 region. The author suggests, in exjjlanation 

 of this deficiency of cleavage on the mar- 

 gins, that these parts were folded under a 

 less load than the more central portions and 

 were earlier lifted above the line of com- 

 pression. The limestones are characterized 

 by suture-like cracks so well known in lime- 

 stones and marbles the world over. These 

 are held by the author to be due to solution 

 under pressure, and evidence in favor of this 

 view is adduced. 



The discussion of the metamorphism is 

 perhaps the least important section of the 

 book, and contributes little of importance 

 to the general subject. 



The discussion of the cause of mountain 

 uplift and folding is chiefly interesting for 

 the clear and concise statement of the ex- 

 pansion theory as an adequate explanation 

 of the origin of mountain structures and 

 plateau upUfts. The advantages of this 

 theory over the docti'ine of the earth's con- 

 traction under secular cooling are clearly 

 set forth. The doctrine of secular contrac- 

 tion fails to give an adequate explanation 

 of the phenomena of volcanologj^ ; it does 

 not account for the distribution of the force 

 of gravity ; and it involves too great a 

 shortening of the earth's radius. The ex- 

 pansion theory does not have these objec- 



