178 Revieics — A New Rock- Classification. 



Our criticism, so far of a merely destructive kind, will be made 

 more intelligible if we may be allowed to indicate rougWy the lines 

 on which, as we think, a real advance may be looked for. Instead 

 of beginning at the top and dividing down, we would begin at the 

 bottom and build up. The unit of classification would be the 

 sufficiently definite entity usually spoken of as a rock-type. The 

 necessity for a given type being once established, and the characters 

 of the type clearly defined, it should receive a name, preferably 

 derived, as is customary, from the type-locality ; and thereafter any 

 attempt to alter the definition, or to extend the name beyond it, 

 should be firmly resisted. If this proviso could be secured, the 

 often-heard objection to the multiplication of names would become 

 merely an objection to the increase of knowledge. Concurrently 

 with the settlement of known types and the investigation of new 

 ones, we should expect to gain a clearer insight into their mutual 

 relations from the genetic point of view, and the information thus 

 acquired, in the field as well as in the laboratory, would enable 

 us in due time to group the types into a natural system expressing 

 our knowledge of their chemical, geological, and geographical 

 relationships. In such a system there would be no finality ; indeed, 

 its changes would be in some degree a measure of the progress of 

 petrological science. The conception is one far removed from that 

 embodied in the scheme now before us. It is true that a natural and 

 an artificial system might conceivably exist side by side, each being 

 employed for its appropriate purposes. Such a dual classification is 

 evidently contemplated by the authors, when, for instance, they 

 expressly leave the terms ' family ' and ' series ' free for use in 

 a petrogenetic connection. It is easy to see, however, that the case 

 of petrology differs in fundamental respects from that of botany, 

 and we cannot doubt that the general adoption of a fixed empirical 

 scheme would greatly hinder the development of a natural classi- 

 fication at some future time. Whether the present bold proposal 

 meets in a convenient manner the immediate requirements of 

 petrography is a different question, and one which will find its true 

 answer in the reception accorded to the scheme by the general body 

 of workers. 



In conclusion, we may state that the exposition of the system 

 by its inventors leaves nothing to be desired in respect of lucidity 

 and precision. It is reprinted, together with an admirable historical 

 survey of rock-classification by Dr. Whitman Cross, from the Journal 

 of Geology. The volume contains certain additions, including 

 a useful glossary, which inspires the reflection that compositors 

 and proof-readers will not be among those who welcome the new 

 terminology. The book seems, however, to be carefully corrected, 

 and is brought out in a form which fully maintains the reputation of 

 the University of Chicago Press. 



A. H. 



