276 Revieios — Prof. T. Sterry Hunt — 



pqr, and the proportions, as defined by the shaded line, of the 

 downward protuberance, or "roots of the mountains," will be 

 about that required for equilibrium. The height, to which the 

 disturbed tract is represented to have been forced up, is no doubt 

 very great. But so little is known of the sort of positions, in 

 which the rocks of elevated regions were left when first raised, 

 that it cannot be a priori pronounced impossible. The laws, which 

 govern the initial positions and height of mountainous regions after 

 great disturbance, are an unsolved problem in geology. It must 

 not be understood, because the mean surface is represented as 

 undulating, that the rocks are asserted to have been plicated in 

 curves parallel to it. Much less is it meant to be implied by the 

 diagram, that the present surface is identical with the original 

 elevated surface, sunken. It will be rather some part of the crust 

 below the sea-level in the diagram, re-elevated and denuded. 



In a few words then. Cleavage is due to an internal movement 

 of the rocks, rendered necessary by the disturbed region having been 

 left, after elevation, in a position too lofty for equilibrium. 'J'his 

 internal movement would have been accomplished by faulting, had 

 not the friction been too great, owing to pressure, to allow of sliding 

 along surfaces of separation.^ Viscous shearing therefore performed 

 the office, and produced cleavage surfaces. 



la IE "V I E -v^ S. 



I. — The Geological History of the Serpentines, including 

 Studies of Pre-Cambrian Books. By Thomas Sterry Hunt, 

 M.A., LL.D., F.E.S. From the Trans. Roy. Soc. Canada, vol. i. 

 sec. iv. 1883. (Montreal, Dawson, Brothers.) 



THIS communication is, to a certain extent, one of the results of 

 the International Congress of Bologna (1881), where great 

 interest was evinced in the question of Serpentines, a special meeting 

 having been held under the presidency of Dr. Hunt, in which both 

 French and Italian geologists of high standing took a part. 



Serpentine, even by itself, is a very slippery subject, and when 

 it is allowed to shade off through innumerable ophiolites, ophicalcites, 

 etc., into the sparry and earthy carbonates on the one hand, and 

 through the serpentinized gabbros into the felspar-pyroxene rocks 

 on the other, there is endless scope for mystification. Moreover, 

 if to all this be added certain " Studies of Pre-Cambrian Rocks," 

 we have the elements of a considerable amount of that geological 

 theorizing for which the writer is so celebrated. Nevertheless we 

 are bound to admit that many of Sterry Hunt's ideas, both in the 

 Serpentine and Pre-Cambrian connection, are not so much regarded 

 in the light of mere theories as they were a few years ago. He has 

 lived to see the old doctrines of the regional metamorphism of 



^ This appears to be the rationale of M. Tresca's experiments, by which he proved 

 that solid, ductile, or pulverulent bodies, can, without changing their states, flow in 

 a manner analogous to that of liquids, when sufficiently great pressure is exerted on 

 their surface. 



