6o PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



fication of once glassy acid lavas, and we are at present unaware 

 how far this alteration has extended. Again, there are many de- 

 ficiencies in our terminology which need to be supplied. The 

 majority of our names for members of the metamorphic group 

 imply the existence of foliation, yet we not unfrequently meet with 

 rocks in which, though no orientation of the mineral constituents 

 exists, the structure differs generally from those characteristic of 

 igneous rocks, and agrees with those characteristic of metamorphic. 



It is no part of my present purpose, for it would lead too much 

 into a discussion of minute details, to propose a system of nomen- 

 clature. I shall accordingly content myself with indicating the 

 principles on which such classification should be founded. Eor this 

 purpose, as I pointed out last year, we may regard the igneous rocks 

 as primordial, and the others as derivative. The latter originate from 

 the former, either directly by some process of detrition, the result 

 being what are called the clastic rocks ; or indirectly by separation of 

 constituents, mainly by chemical action, and subsequent precipitation. 

 To the second of these groups belong (for instance) such rocks as 

 beds of anhydrite, gypsum, and rock-salt, with certain limestones and 

 XDossibly some dolomites. To the first belong the great bulk of the 

 stratified rocks. The igneous rocks also may undergo alteration, 

 either by simple chemical change, as in the instances quoted above, 

 or by mechanical crushing, followed by chemical changes. The 

 results of the latter process vary much; sometimes the previous 

 history of the rock is evident, sometimes it is dubious. Thus some 

 porphyroids are simply crushed rhyolitic rocks, in which a filmy 

 micaceous mineral has been subsequently developed along the rude 

 planes of cleavage ; some " schalsteins " are probably compact 

 basalts that have undergone the same treatment ; some amphibolites 

 and hornblende-schists are crushed doleritic rocks, in which greater 

 subsequent changes have occurred. As we shall presently see, any 

 igneous rock may be crushed in situ, and the result of subsequent 

 mineral change may almost — nay, locally, may wholly — efface its 

 original constitution. 



The clastic rocks may be produced either by the explosive action 

 of volcanoes, or by the detritive action of rain, rivers, sea, &c. The 

 constituents of the former will, of course, be mainly of igneous origin. 

 The latter will be the ordinary arenaceous, argillaceous, or other 

 sediments, which are so familiar to us that I need not delay over 

 them. Lastly there are the rocks whose origin is mainly organic, 

 the accumulations of the remains of plants, or the more solid parts of 

 animals. 



These rocks, whether of clastic or organic origin, also undergo 

 alteration varying in amount and in its nature. In the first stage, on 

 the chemical side, there are the micro-mineralogical changes which 

 result in the consolidation of material once incoherent, and the partial 

 obliteration of original structure ; on the mechanical side there is the 

 production of cleavage. In the second stage, there is the entire or 

 almost entire rearrangement of the constituents of the rock, so that, 

 as a rule, we can only infer its origin by processes of inductive 



