9 



If now we direct our attention to the rocks of which the earth's crust is com- 

 posed we have no difficulty in recognising the fact that a large proportion of 

 them have been produced by the agencies above referred to. We recognise 

 rocks that have been formed by surface agencies the stratified rocks ; and 

 others that have been formed by igneous action the massive rocks and their 

 tuffs. We recognise also metamorphosed representatives of these two classes. 



When, however, we have classified all those rocks about the origin of 

 which there is no doubt, we are obliged to recognise the existence of others 

 which do not fall readily into our scheme of classification. The crystalline 

 schists of archaean age are the most important of these. Some geologists 

 maintain that these schists are portions of the original primitive crust, (1) 

 and that they should be classed with the igneous rocks ; (2) others 

 that they are chemical precipitates from a primordial ocean ; others 

 that they are the result of a peculiar kind of metamorphism, diagenesis, 

 acting upon the sediments or precipitates of such an ocean ; (8) others 

 that they are the result of ill-defined metamorphic processes operating 

 upon ordinary sedimentary rocks, without seriously disturbing, or in any way 

 obliterating, the original order of stratification ; (4) and others that they are the 

 result of regional and contact metamorphism operating upon complex systems 

 of massive and stratified rocks. (5) 



Having regard to the great difference of opinion which exists between 

 competent observers it appears desirable to establish a third group for the 

 rocks of doubtful origin. (6) This group will include all those rocks which can- 

 not be referred to a place in the above scheme of classification. It may be 

 asserted with confidence that the advance of science will tend to reduce 

 this group. 



It will be understood, from what has already been stated, that the author 

 does not recognise the independent existence of a third group of rocks, to which 

 the term metamorphic is applicable, of equal importance with the well 

 defined groups which are somewhat imperfectly designated by the terms 

 stratified and massive ; or aqueous and igneous. 



In the present work, the igneous, or massive, rocks will first be con- 

 sidered, then the aqueous or stratified, and lastly, the rocks, which must at 

 present be placed under the head of incertce sedis. Metamorphic rocks, so far 

 as they can at present be recognised, will be described in connection with the 

 original rocks of which they are the metamorphosed representatives. 



(1) De Lapparent, Traite de Geologic, 1st Edit., page 612. 



(2) Roth. Beitrage zur petrographie der plutonischen Gesteine. 



(3) Giimbel. Geognostische Beschreibung des Ost-bayerischen Grenzgebirges. Gotha, 1868. 

 833845. 



(4) Dana. Manual of Geology. 



(5) Lehmaun. Die Entstehung der alt-krystallinischen schiefer Gesteine. Bonn, 1884. 



(6) See Lossen, Op. cit. p. 501. 



