504 Hutton — Classification of Rocks, 



others which, I hope, will not be open to the same objection. The 

 usual mode of making the Metamorphic, Plutonic, and Volcanic 

 rocks form separate classes is, in my opinion, highly artificial, as it 

 separates widely many rocks closely connected both chemically and 

 geologically. I have therefore used these terms to separate only the 

 sections chemically related into allied groups, and have taken the 

 original formation of the rocks themselves as the basis for dividing 

 them into the larger classes and divisions. 



I prefer the terms " subfBrial" and ''subaqueous" to " aBrial" 

 and " aqueous," as they better describe the position of the rock when 

 being formed, and the complimentary term "subterranean" implies 

 no theory ; so that although some geologists might maintain that 

 granite is an aqueous rock, none would insist upon its being '•' sub- 

 aqueous," or produced in its preseM form underwater; and I also 

 think that none will be inclined to deny that the rocks in the " sub- 

 terranean" class have received their final form, in which we now 

 see them, below the earth. It may, indeed, be objected that the 

 Volcanic groups were solidified, and therefore formed above the 

 earth ; but, to my mind, these rocks were chemically formed in the 

 Volcano, and the fact of their having cooled on the surface as lava, 

 or below the earth as dykes, makes no essential difference, although 

 it may constitute varieties. The case is quite different with the 

 ejected tuffs and tufas. When they were below the earth they were 

 lavas, and it was not until they were ejected, and mixed with other 

 substances, that they became tuffs. 



The division of the subasrial class that I have called "conflated" 

 is a very important one in many parts of the world, and ought not 

 by any means to be omitted. Here, for instance, in New Zealand, 

 it sometimes forms hills 500 and 600 feet high, rudely stratified, 

 and bound together with hard ferruginous cement, and often dis- 

 playing beautiful examples of cross bedding. 



In the "Ejected" and "Tufaceous" divisions it will be noticed 

 that I have confined the word "tuff" to subserial accumulations, 

 and "Tufa" to subaqueous ones. Geologically this distinction is 

 important, for besides the diiferent relations of land to water that 

 they show, tuffs always mark the site of a volcano, while tufas may 

 be found many miles distant from the point of erui^tion. I should 

 consider the mud streams that sometimes run down the sides of a 

 volcano as a variety of tuff. 



The metamorphic groups of the Tufaceous division may be open 

 to objection as too theoretical, but if we allow volcanic action to 

 have taken place during Palaeozoic times, tufas must have been 

 formed, and some must now exist in a metamorphosed condition ; so 

 that supposing it shown that some of the rocks placed in these 

 groups had not a tufaceous origin, still the error would be in the 

 arrangement of the rocks in the system and not in the system itself. 

 The tufaceous rocks are, no doubt, nearly as much entitled to the 

 name "ejected" as the suba3rial tuffs, but I have thought it better 

 that each of the divisions should have a distinct name, and the rocks 

 belonging to the tufaceous division are often mixed with sand, etc., 

 and so are not altogetlier ejected. 



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