W. N. Benson — Origin of Serpentine. 699 



in Suess's statement that the "green rocks form sills in 

 dislocated mountains, which sometimes follow the plane 

 of bedding, and sometimes the plane of movement" 

 (Suess, 1909). The association of normal serpentine 

 with limestones is accidental except in so far as these 

 comparatively weak structures may have determined the 

 plane of intrusion (Trabucco 1896?). 



HI. The Process of Serpentinization. 



The nature of the original rock from which the ser- 

 pentine was derived having been thus ascertained, the 

 discussion turned to those problems with which this 

 paper is specially concerned, namely, actual process of 

 serpentinization, the method, time and place in which the 

 hydration occurred. Most writers prior to 1899, such as 

 Teall(1888), McMahon (1890), and Roth (1869, 1893), 

 but with the exception of Daubree (1879), had referred 

 the process to the action of percolating meteoric waters, 

 i. e. to atmospheric weathering. In recent times Crosby 

 (1914) also held this, and Julien (1914) has put forward 

 in some detail his view concerning the development of 

 serpentine by superficial action followed by a more 

 deeply seated change. This interesting discussion may 

 here be summarized. Julien divided into three stages 

 the processes leading to the formation of antigorite, by 

 which term he implied the mineral species, the composi- 

 tion of which is expressed by the formula H 4 Mg 3 Si 2 9 , 

 as distinct from the serpentine-rock, which is a mix- 

 ture. (Whether this usage of the term " antigorite ' ' 

 is permissible is another matter.) The three stages 

 are: — "decay, the result of operations within the belt 

 of weathering, disintegration and extreme hydration. 

 Among the more important products are colloid magne- 

 sian silicates of the first type (colloid deweylite, sepio- 

 lite), magnesium, oxide, hydrate and giobertite, besides 

 various forms of ferrous and ferric hydrate, hydro-car- 

 bonates, etc. ; alteration to express the interchange and 

 consequent new formations, with great loss of water, 

 which take their birth in a more deeply seated region, 

 the common products of which are the magnesian hvdro- 

 silicates of the second type (talc, antigorite), hardened 

 deweylite, forms of limonite, turgite, hematite, etc. ; and 

 decomposition to express molecular dissociation, still 

 more complete interchanges, and still greater, to com- 



