METHYLOTIC ORIGIN OF OPHITES. 41 



view, it has been generated out of minerals containing silicate of 

 magnesia and oxide of iron, such as hornblende and augite : it is 

 rocks composed of minerals of the kind, and which are frequently 

 methylosed, that form the principal repositories of peridote. 

 We feel certain that Zirkel will not be able to explain the origin 

 of the " crystalline limestone " which encloses the pseudomor- 

 phosed crystalloids without availing himself of the aid of 

 methylotic processes. Moreover it may be considered that the 

 presence of peridote in rocks which have undergone chemical 

 changes is a barrier to this mineral being regarded otherwise 

 than as a concomitant product of such changes. 



For our part, we are acquainted with sufficient evidence to 

 sustain us in the conclusion that serpentine, instead of origi- 

 nating from any one mineral in particular, is polygenetic. The 

 numerous examples we have lately found around Galway (and 

 their number increases by every fresh examination) of serpen- 

 tinized granite, diorite, porphyry, and feldsyte, may be safely 

 accepted as completely confirming the view, held by Rose and 

 Bischof, that serpentine may be generated out of widely different 

 rocks and minerals, admitting at the same time the magnesian 

 constituent to have been derived from foreign sources. 



We are even not averse to the view that serpentine or ophite, 

 hitherto limited in its derivation to silacid rocks, has in some 

 instances been produced from chemical changes in carbacid 

 deposits. From what may be observed in the dolomitic lime- 

 stone in immediate contact with the diorite of Conzocoli (at which 

 junction, a few hundred feet up on the flanks of this mountain, 

 one may sit on both rocks at the same time), near Predazzo, 

 no doubt can prevail that the layers and patches of serpentine 

 present in the limestone are a local development, due to dis- 

 charges of silica, in some form or other, from the adjacent 

 igneous rock*. 



This is the only instance that has come under^our notice of a 



* On a former occasion we made known that the Isle-of-Skye Jurassic 

 ophite is in some places, as near Torrin, lamellated with the same mineral 

 substances, and in a similar style, as "eozoonal " ophite: specimens collected 

 by one of us at the Conzocoli junction are precisely similar. It is noteworthy 

 that the serpentine is occasionally replaced by a mineral substance, seemingly 

 loganite (as at Burgess in Canada), the latter occurring interlamellated with 

 hydrous dolomite (predazzite). 



