DYNAMICAL GEOLOGY. 313 



under several headings : curvature, plication, crush, shear, 

 cleavage, distortion of rock-material and of fossils. He 

 opposes Thurmann's idea that the rocks are primarily 

 plastic and remain so during the mountain-movements, and 

 assumes that the rocks of our mountain-chains have been first 

 consolidated and afterwards altered during the crust-movements; 

 the alteration might be accompanied by fissures and faults or 

 might take place without any fracture, both modes of trans- 

 formation being quite independent of the physical and chemical 

 constitution of the rocks. Alteration without fracture only 

 occurred at great depths, and was most frequent in the older 

 rocks. According to Heim, the essential conditions for such 

 alteration are the presence of a heavy superincumbent load of 

 rock, and the action of pressures from all sides upon the rock- 

 particles, so that even the most brittle mass of rock would 

 be converted into a state of latent plasticity. The work done 

 by horizontal pressures is the great truth which Professor Heim 

 seeks to inculcate. He brings forward numerous observations 

 to prove the passive behaviour of the "Central Massives" during 

 the upheaval of the present Alpine system. In opposition to 

 Studer's idea that the massives had represented active local 

 centres of disturbance, Heim points out that the crystalline 

 rocks present in these areas themselves show deformation and 

 alteration explicable only upon the assumption that they had 

 suffered no less than the rocks in the northern and southern 

 zones of the Alps from a system of horizontal pressures common 

 to the whole Alps. In Professor Heim's opinion, the individual 

 forms of the Central Massives as lenticular or fan-shaped arches 

 or simple domes had been determined by modifying local in- 

 fluences during the epochs of Alpine upheaval, but had no 

 connection with volcanic subterranean forces. On the con- 

 trary there is, according to Heim, no field evidence whatsoever 

 that the igneous rocks of the Central Massives exerted forces 

 of compression upon the sedimentary strata in contact with 

 them. 



Heim therefore agrees with the general results of Suess, 

 and explains mountain-making as a consequence of nuclear 

 contraction, crust-subsidence, and the complex action of 

 horizontal strains through the layers of the crust. He 

 calculates that the plication of the Alps has reduced the 

 breadth of that portion of the crust by a distance of about 

 seventy -four miles; hence the crust contraction would seem 



