•Si Fo7'bes — On aUeged Hydrotkermal 



The cont^iction on tte writer's mind is not only that the perhaps 

 should have been omitted, but also, that the analyses should have 

 preceded, and not followed, the enunciation of views which these 

 analyses, when made, may not improbably entirely annihilate. 



How, may it be fairly asked, can any educated man, whether 

 geologist or not, be expected to believe that greywacke may be con- 

 verted into granite ; unless, first of all, he is shown by chemical 

 analysis that you have present in the first, the chemical elements 

 requisite for the formation of the latter, or if not, that you have 

 a rational mode of explaining how any deficiency in component 

 parts has been supplied, or any surplus removed. 



If such tangible data and explanations cannot be given, then all 

 like hjrpotheses must share the fate of the old alchemical visions of 

 transmutation, and it is but waste of time, thought, and energy to 

 place them before a rational public. 



Supposing, for the sake of argument, however, that data are given 

 which showed that there was no obstacle, from a chemical point of 

 view, to believing that a certain bed of greywacke had been con- 

 verted into granite, then these very facts would be most conclusive 

 arguments against Mr. James Geikie's assertion that this greywacke 

 had not only been converted into granite, but, also into diorite, ser- 

 pentine, porphyrite, etc., etc. ; all rocks, differing essentially in che- 

 mical and mineralogical composition, not only from granite, but 

 from one another. 



(2) Mineralogy. — Although the mineralogy of these memoirs will 

 be considered, chiefly in conjunction with the petrology, still some 

 points, purely mineralogical, require more definite explanation than 

 the author gives ; for example, with respect to felspar, on the study 

 of which so much of his results are based, is not the mineralogist 

 entitled to demand (seeing that this name is only a generic one, 

 including mineral species widely differing from one another in 

 chemical composition, as orthoclase, oligoclase, albite, anorthite, 

 labradorite, etc.) that the author should state what mineral he 

 actually alludes to, or if not crystallographer enough to do so, at 

 least state whether he writes of potash, soda, or lime felspars ; for 

 how else can any opinion be formed as to whether the species of 

 felspar is at hand which is characteristic of the rock which he 

 supposes is formed by his metamorphic agency. For example : sup- ■ 

 posing the felspar alluded to was a lime felspar, then the crystalline 

 rock formed by the metamorphic action could not be a granite, for 

 . we know that lime felspar is never a normal constituent of any 

 granite. 



At page 516, Quart. Journ., under the head of " Amygdaloid," it is 

 stated that " the matrix consists of a paste of felspathic matter, with 

 here and there a variable admixture of magnesia and lime." Miner- • 

 alogists would find this somewhat difficult to explain, as native 

 lime is never found in the mineral kingdom, and native magnesia 

 only occurs in some active volcanoes, as the extremely rare mineral 

 .:periklase,^ so that neither of these substances are likely to have been 

 1 A hydrate of magnesia is also knowu as brucite. 



