542 T. H. Holland — Rock-icentJiering and Serpentinization. 



Professor Merrill has now advanced farther evidence in support 

 of my first point by showing that there are essential chemical 

 ■differences between serpentine and the weathered products of 

 magnesian rocks. ^ Althougli. however, we are agi'eed, Professor 

 Merrill and I, that serpentine, as one instance of a secondary 

 hydrous mineral, has been wrongly ascribed to weathering, we 

 find ourselves at variance when we pass from destructive criticism 

 to the creation of a more satisfactory substitute for the old ideas. 



As the presentation of objections to an opponent is generally an 

 easier problem than proving one's own case, I will commence with 

 a criticism of Professor Merrill's theory. According to his view, 

 " serpeutinization is a deep-seated process- due to waters or vapours 

 coming from considerable depths, and it may be even constituents 

 of the magmas at the time of their intrusion," " The almost complete 

 absence of oxidization products in fresh serpentine is," he says, 

 " indicative of this." ^ 



This last clause, given as a reason for the conclusions indicated 

 in the previous part of the same paragraph, is the one which I would 

 iirst of all criticize, and against which I will produce, besides the 

 statements of Eoth, the testimony of a very reliable authority to 

 ■show that oxidation is a constant, and presumably essential, phase 

 lin the formation of sei'pentine from olivine : — 



" The serpentinous alteration [of olivine] takes place along the 

 irregular curvilinear lines of fracture, and under favourable conditions 



-continues until the transformation is com[)lete The 



iron, which existed as protoxide, is further oxidized, and crystallizes 

 out along lines of fracture as magnetite or hematite, or in the 

 liydrous sesquioxide form known as limonite." 



" In this process of hydration the combined iron becomes converted 

 into the sesquioxide form." 



"The mineral olivine, an anhydrous ferruginous silicate of mag- 

 nesia, passes over into serpentine by a simple process of hydration, 

 ^nd a more or less complete change of its combined iron from the 

 ferrous to the ferric state." 



So we learn from the fascinating " Treatise on Eocks and Rock- 

 weathering." * And in this matter its author is in perfect agreement 

 with other earlier authorities. The limited amount of oxidation may 

 be claimed to be as consistent with the submarine as the subterranean 

 theory, but as it is not part of the evidence on which I base the 

 former it may be left out of the present argument. 



" The action of waters or vapours coming from considerable 

 depths " has often been referred to by earlier writers as the cause of 



1 That serpentine loses "svater by exposure to the weather is not in itself, however, 

 a proof that the previous hydration of the olivine was not effected by atmospheric 

 water. Laterite, which is unquestionably a product of true, though peculiar, 

 weathering, loses water on exposure even to a humid atmosphere. 



2 W. Lindgren in 1896 also referred to serpeutinization as a deep-seated process, 

 ^vhich, he said, should not be referred to weathering : 17th Annual Eeport, U.S. 

 •Geol. Surv., pt. ii (1895-6), p. 92. 



3 Geol. Mag., August, 1899, p. 356. 

 * pp. 24, 25, 97, 158, and 159, 



