‘PartIl.§v.] CLASSIFICATION OF ROCKS. — 109 
the rocks, and consists of two divisions, lst, Stratified, embracing all 
- the aqueous and sedimentary with part of the less altered metamorphic 
rocks; 2nd, Unstratified, nearly conterminous with the term igneous, 
since it includes all the eruptive rocks. Further subdivisions of this 
series have been proposed according to differences of structure or 
texture, as porphyritic, gramtic, &e. These geological subdivisions, 
however, ignore the chemical and mineralogical characters of the 
rocks, and are based on deductions which may not always be sound. 
Thus rocks may be included in the igneous series which further 
research may show not to be of igneous origin; others may be 
classed as metamorphic, regarding the true origin of which there may 
be considerable uncertainty. A further system of classification based 
upon relative age has been applied to the arrangement of the eruptive 
rocks, those masses which were erupted prior to the close of Secondary 
time being classed as “older,” and those of later date as “ younger.” 
This system has recently been elaborated in great detail by Michel- 
Lévy, who maintains that the same types have been reproduced néarly 
in the same order in the two series, though basic rocks, often with 
vitreous characters, rather predominate in the later. But it can be 
shown that some rocks occur in both series, and though there are 
undoubtedly well-marked differences between some Tertiary and pre- 
Tertiary eruptive rocks, it may be doubted whether this classification 
is not too ingenious and artificial.* 
Though no classification which can at present be proposed is 
wholly satisfactory, one which shall do least violence at once to 
geological and mineralogical relationships is to be preferred. Avoiding 
therefore all theoretical considerations based on deductions as to the 
origin of rocks, we may conveniently make use of the broad dis- 
tinction between Crystalline (including vitreous) and Clastic or 
Fragmental rocks. The former are, Ist, stratified, including chiefly 
chemical deposits, such as limestones, dolomites, sinters, &c.; 2nd, 
schistose, embracing most of the so-called metamorphic rocks; 8rd, 
massive: this series is nearly coincident with the old division of 
Igneous Rocks. The Clastic or Fragmental rocks are formed either 
of the débris of older rocks, or of the aggregated remains of plants or 
animals. In some cases, as for example, in limestones of organic 
origin, subsequent alteration gradually effaces the fragmental 
structure, and superinduces a true crystalline internal arrangement. 
Hence along certain lines fragmental rocks pass gradually into the 
stratified crystalline series. | 
It must be kept in view that in this proposed system of 
classification, and in the following detailed description of rocks, 
many questions regarding the origin and decomposition of these 
mineral masses must necessarily be alluded to. The student, however, 
? See on this subject, J. D. Dana, Amer. J. Sci. xvi. 1878, p. 336. Compare also Michel- 
Lévy, Bull. Soc. Geol. France, iii. 3rd ser. p. 199, vi. p. 173. Fouqué et Michel-Leéevy, 
op. cit. p. 150. Rosenbusch, Mik. Physéog. ii. On the classification of compound silicated 
rocks, see Vogelsang, Z. Deutsch. Geol. Ges. xxiv. p. 507, and for an incisive criticism of 
too merely mineralogical classification, Lossen, op. cit. xxiv. p. 782. 
