W. N. Benson — Origin of Serpentine. 719 



have been scarcely investigated, much less exhausted. It 

 is interesting to note again in the Tyrolese instance the 

 presence of carbonates in the olivine-antigorite veins 

 that traverse the serpentine. 



vi. Where geological evidence is available of the 

 period of hydration, the serpentine seems to have been 

 formed at a comparatively short time after the intrusion 

 of the ultrabasic rock. It is true that Crosby (1914) 

 considers that serpentinization of ancient peridotites 

 may be even now in progress at some depth in the earth, 

 and suggests that the high frequent relief of such ser- 

 pentine masses may be due to the steady upthrusting of 

 the rock, as it expands during combination with circulat- 

 ing waters in the crust ; but this explanation of the relief 

 of serpentine areas seems unnecessary. It has long been 

 recognized that the chemical stability of the serpentine 

 molecule gives it great power of resisting weathering, 

 though the rock yields readily to erosion. (See e. g. Hunt 

 1883.) If we study the conglomerates that were formed 

 shortly after the intrusion of ultrabasic rocks, we find 

 that sometimes they contain pebbles of normal serpentine, 

 so similar to that occurring in the great rock-masses 

 from which they were derived, that it is scarcely possible 

 that they could have been formed under different condi- 

 tions, the one as a large deep-seated plutonic mass of 

 peridotite, the other as a pebble of peridotite included in 

 a conglomerate, where it would encounter strongly oxi- 

 dizing waters. It is therefore most probable that the 

 plutonic mass had been hydrated to serpentine before the 

 pebble was torn from it. As instances the following may 

 be cited: Serpentine-pebbles appear in the Silurian 

 rocks near Ballantrae in Scotland, derived from ultra- 

 basic rocks intrusive into the Ordovician rocks (Peach 

 and Home 1897). A pebble of serpentine, probably 

 derived from the peridotite that was intruded in Middle 

 Carboniferous times, occurs in the lower Permo-Carbonif- 

 erous beds of New South Wales (David 1907). It is 

 quite clear that the carbonation of the Carboniferous ser- 

 pentine in the Warialda District of New South Wales 

 occurred before the deposition of the overlying Juras- 

 sic sandstone (Benson 1916). The ultrabasic rocks of 

 Upper Jurassic age have furnished the bowlders of ser- 

 pentine which form a great part of the Middle Creta- 

 ceous series of Bosnia. [Such at least is Katzer's view 



