No. 378.] REVIEWS OF RECENT LITERATURE. 465 
that the “ Sigillaria are to be regarded as a central generalized 
group, from which, in regard to structure and affinities, various 
genera radiate towards Cycads and Conifers on the one hand and 
Lycopods and Equisetums on the other.” DPPP 
PETROGRAPHY. 
The Classification of Igneous Rocks. — Messrs. Iddings' and 
Cross” have contributed two interesting articles on that most attract- 
ive of all petrographical problems, the classification and naming of 
igneous rocks. Although attacking the subject from entirely different 
standpoints, both authors nevertheless reach approximately the same 
conclusions. Cross declares that “the impossibility of setting up an 
all-embracing natural classification of igneous rocks is not due to 
ignorance. It comes from the nature of the rock. The more we know 
the less shall we be able to include all relations in one classification.” 
Iddings states “that a systematic classification of all kinds of 
igneous rocks cannot be put on the same basis as a philosophical 
treatise of the subject-matter of petrology, which takes cognizance 
not only of the material character of rocks, but also of the laws 
governing their production, eruption, mode of occurrence, and solidi- 
fication, as well as their subsequent alteration.” 
Iddings discusses critically, with the aid of diagrams, the composi- 
tion of igneous rocks, as indicated by nine hundred and fifty-eight 
analyses, and shows that no chemical classification will exhibit the 
true genetic relationships existing between different rock types, and 
that a mineralogical classification is likewise useless for this purpose. 
It is, therefore, of no avail to attempt a genetic classification of rocks - 
if it is desired by the classification to group together those rocks that 
have like characters, in order that they may receive a common name. 
The present classification, and the nomenclature to which it has 
given rise, are both unsatisfactory. The need for a new nomencla- 
ture is especially pressing, and yet “the condition of our knowledge 
at present is scarcely such as to warrant the immediate attempt to 
create a systematic nomenclature.” 
The point of Cross’s paper is to the effect that the present unsatis- 
factory condition of rock classification is due to the fact that too many 
l Iddings, J. P. On Rock Classification, Journ. of Geol., vol. vi, p. 91. 
2 Cross, W. The Geological versus the Petrographical Classification of Igneous 
Rocks, Journ. of Geol., vol. vi, p. 79. 
