52 Forbes — On alleged Hydrothermal 



whatever, before he has submitted the data, upon which they are 

 founded to as strict and impartial a scrutiny as his own knowledge 

 of the subject permits. 



Of such a scrutiny a short abstract is now given, and for con- 

 venience it is divided into several heads : — 



(1) Chemistry. — Although Mr. Geikie admits " that the labours of 

 the chemist have been invaluable," and that he has "no disrespect 

 for the work of the laboratory," he generally finds a hut or perhaps 

 to qualify his praise, and to show how much more dependence ought 

 to be placed in the field observer. 



It is interesting, however, to observe how much chemistry he has 

 introduced into his first memoir, and how he endeavours to base his 

 entire conclusions thereon. From the style of this memoir (but for 

 its errors) it might have been written by a chemist, but certainly 

 not by a mineralogist or petrologist. 



Every chemist knows that magnesia is the oxide of the metal 

 magnesium, and that it possesses no colour in itself, nor even the 

 property of communicating colour to its compounds, unless white be 

 regarded as a colour ; and that, moreover, the minerals containing 

 magnesia are invariably colourless, unless some other element is also 

 present capable of communicating a colour to them. 



Mr. James Geikie difiers from chemists on this point, and appears 

 to have made the interesting discovery that magnesia is a colouring 

 body and has the property of colouring minerals green. Thus (p. 

 515, Quart. Journ.), he states that the magnesian matter colours the 

 rocks green, and when speaking of the greywacke of Peebleshire 

 states, (p. 518,) " The beds have a greenish tinge from the abundance 

 of magnesian matter which they contain," and all throughout the 

 memoir it is evident that he looks upon this new colouration test for 

 magnesia (greenness) as a means of assuring oneself of its presence 

 in rocks. 



A well known mineralogist, when looking over Mr. James Geikie's 

 memoir along with the writer, expressed his opinion that this mis- 

 conception was one likely to occur to a field observer from confound- 

 ing magnesia with serpentine — ^that latter mineral, when pure, being 

 commonly green, and its chemical analysis showing it to contain some 

 40 per cent, of magnesia. The -field observer might therefore infer that 

 the colour was due to the magnesia it contained, but the chemist 

 could have informed him it was due to iron. 



Again, we find (p. 518, Quart. Journ.), when alluding to the un- 

 altered strata, it is stated, " In places they are parti-coloured, showing 

 yellow and green blotches owing to the decomposition of alkaline 

 matter with which the beds appear to be more or less charged." 

 This paragraph may be understood by the author, but neither the 

 writer, nor several eminent chemists, before whom he has laid it, 

 can pretend to explain the peculiar chemical action alluded to in this 

 sentence, and as regards the assertion contained in the latter part of the 

 same, that all the beds appear to be more or less charged with alka- 

 line matter, it may be stated at once, that the external appearance of 

 such rocks could not prove to any chemist, mineralogist, or geologist. 



