OCR Text |
Show BASIS OF THE CLASSIFICATION OF VOLCANIC EOCKS. 85 assumed correlations are, so far as we can discern them, seen to be only very partial and imperfect. Slill we may hold that, for the time being, the best classification is the one which expresses the largest number of facts and relations hitherto ascertained, and we may advantageously adopt such a classification in preference to any other, though conscious that it fails to bring into recognizable order some outstanding facts and relations which we are compelled for the present to look upon as anomalies. In proposing a system of classification of volcanic rocks, I shall endeavor to conform to the foregoing conceptions as to the purposes and scope of any or all classifications. Strictly speaking, I can pretend to nothing more than the most convenient and accurate expression which the nature of the case may admit, of the state of my own knowledge and convictions concerning the properties and relations of volcanic rocks. Holding that all classifications are ephemeral, merely indicating the instantaneous phases of advancing knowledge, it is fully admitted to be an artificial one for the most part, and is natural only so far- as nature has been truly discerned and expressed. The object in presenting a new classification instead of selecting and adopting an old one is to give precision to the terms employed, and to lay down from the beginning a systematic statement of the views entertained regarding the affinities of the various kinds of eruptive rocks so far as known and understood by the individual writer. Not only does there seem to be no impropriety in any or every writer expressing as accurately and systematically as possible his own views of such relations and affinities, but it is rather incumbent on him to do so, and in no way can this be accomplished so compendiously as by a scheme of classification.* In a classification of volcanic rocks, the facts which it is desirable to formulate and arrange are, first, those having reference to the physical con- *I may advert here to a malpractice of some writers, who take advantage of slight pretexts to coin new names for slightly-altered divisions of old groups. A new name is always an inconvenience, even though it may be necessary; unless, indeed, it be a purely descriptive one, conveying at once its significance or giving some conception of its meaning to one who hears it for the first time. Thus, the introduction of such names as protogene, elvanite, nevadite, miascite, &c, entails the necessity of much labor and effort to fix in the memory their meaning, all of which might have been avoided and every useful purpose subserved by using the terms hornblendic granite, quartz porphyry, granitoid rliyolite, nephelin syenite, &c. Irrelevant terms like the first may be very convenient to the writer or speaker, but they are very inconvenient to the reader or hearer. Inasmuch as all classifications are evanescent and constantly shifting, it is manifestly desirable to make them as easily intelligible as possible. |