Revieivs — The Origin of Serpentine. 327 



II. — Tbe Origin of Serpentine : a Historical and Comparative 

 Studt. By W. N. Benson. Amer. Journ. Sci., vol. xlvi, 

 pp. 693-731, 1918. 



FOR some years past Professor Benson has made a comprehensive 

 study of serpentine rocks, arising from his work on the Great 

 Serpentine Belt of New South Wales. He has had the advantage of 

 working at Cambridge, with the advice and assistance of Professor 

 Bonney, and has also visited many European localities and con- 

 sulted some of the leading Continental authorities and examined 

 their material. The results of his investigations are brought 

 together in this valuable paper, which begins with a short but clear 

 account of the historical development of the views of the leading 

 petrologists on the genesis of serpentine. Starting from the now 

 universally accepted view that serpentine rocks are altered dtep- 

 seated peridotites, consisting mainly of olivine and pyroxene, the 

 author shows that in many cases, at any rate, the process of 

 serpentinization is essentially of a pneumatolytic nature, analogous 

 to a certain extent to the formation of greisen, water and carbon 

 dioxide being the chief agents concerned ; coarse-textured veins of 

 serpentine and olivine occasionally found in such rocks show some 

 resemblance to granitic pegmatites. Serpentine with mesh-structure 

 (chrysotile) is usually found in undisturbed regions, while antigorite 

 is specially characteristic of high pressure; the latter is sometimes 

 formed by dynamic metamorphism of the former. It appears that 

 although serpentinization is usually due to the pneumatolytic action 

 of the residues of the same magma that gave rise to the original 

 peridotite intrusion, nevertheless there have often been several 

 intervening intrusions of differentiates from this magma, ranging 

 even to acidic composition, hence the process of serpentinization may 

 appear to be due to a granitic intrusion ; however, geologically 

 speaking, the interval is short and the alteration is completed before 

 the end of the same orogenic and eruptive phase. It is also con- 

 sidered possible that in certain cases a peridotite which has escaped 

 hydration by its own magmatic waters may subsequently be changed 

 to serpentine by the action of deeply circulating epigene waters. 

 At any rate, it is clear from the facts stated by Professor Benson 

 that it is no longer permissible, as has been done by many writers, 

 including the present reviewer, to consider serpentinization as an 

 effect of weathering now in general operation near the earth's 

 surface ; it must for the future be relegated to the category of deep- 

 seated late-magmatic phenomena, which for want of a better name 

 are generally classified under that blessed word "pneumatolytic". 

 R. H. R. 



III. — Bidrag til Einmarkens Geologi. By 0. Holtedahl. Norges 

 Geol. Undersok, No. 84, with English Summary, pp. 314, with 

 21 plates and 2 coloured maps. Kristiania, 1918. 



AS the result of careful field-work extending over nearly six 

 months in the years 1914-17, the author shows that Dahll's 

 classification of the rock- systems of the extreme north of Norway 



