Hutton — Classification of Bocks, 505 



The " Cliemico-organic " division wants more alteration than any 

 of the others, and might, perhaps, be split up into sub-sections and 

 sub-groups with advantage. I have, however, kept them all together, 

 for at present it is impossible to say with certainty whether some 

 of the members owe their origin to organic or chemical causes ; and 

 they form on the whole a very natural group. 



Of the ''Detrital" rocks little need be said. I have placed the 

 Conglomerates and Breccias in a section by themselves, for although 

 they are often very different chemically, they are always the proof 

 of peculiar geological conditions having existed when they were 

 formed. 



The ''Subterranean" rocks I have separated into two divisions 

 that have a very important geological distinction. The "Funda- 

 mental" rocks are those that have been metamorphosed, or, at any 

 rate, have assumed their present appearance, in the place where they 

 were originally formed, so that they generally occupy large districts ; 

 while the ''Migrated" rocks are those that have been forced from 

 the position in which they were first formed, and are now seen, 

 either as dykes or lava streams, only in isolated patches. 



It will be noticed that some rocks occur in two places in the table. 

 This is owing to rocks that we now call under one name having had 

 two different origins. All stratified f el stones, a not uncommon rock 

 in some districts, I look upon as metamorphosed trachytic tufas, 

 while felstone dykes could not have been thus formed ; and although 

 both rocks may have the same chemical composition, they should 

 still have different names. The same may be said with respect to 

 that geological sphinx — Serpentine. The behaviour of massive ser- 

 pentine is so different from that found in dykes, that in a geological 

 classification they must be separated. 



It may be asked why have I removed Clay-slate so far from Mica- 

 schist and its associated rocks ? and why should not Gneiss be classed 

 as a metamorphosed sandstone equally as much as Quartzite ? My 

 answer is that the sub-aqueous origin of Clay-slate, Phyllite, Quart- 

 zite, and Granular limestone is quite apparent, while that of Gneiss, 

 Mica-schist, etc., is not so ; and that the schists are constantly asso- 

 ciated together, while the others are often interbedded with only 

 hemimetamorphic rocks and could not be considered as fundamental ; 

 whilst, on the other hand, to place Gneiss, Mica-schist, etc., among 

 the metamorphic Detrital rocks would be to admit a theory and to 

 break up a geologically natural group ; for if we admit Gneiss among 

 the Detrital rocks why not granite also ? 



It may also be objected that this arrangement separates too widely 

 the lavas from the mechanically formed accompaniments of volcanic 

 eruptions. If, however, we consider the series as circular and not 

 linear, they will be placed in as close juxtaposition as possible, con- 

 sidering the very different aspect and mode of formation of the two. 



Many other objections will no doubt be pointed out, but I think 

 that enough has been said to explain the principles on which this 

 classification of rocks is based ; and I do not wish it to be supposed 

 for a moment that I think that I have completed so difficult a task ; 



VOL. V. — NO. LIII. 33 



