Art. LI.—On the Nomenclature of Rocks.
[Read before the Wellington Philosophical Society, September 17, 1870.]
The nomenclature of rocks is a subject which is involved in considerable confusion, and I regret to observe that geologists, instead of working together and helping to clear it up, are doing their best to make the confusion more confounded, by each one following a different system as far as possible, and thus not only making scientific intercourse difficult, but giving rise to no end of quibbles about word;—also, inducing a very loose way of thinking, and rendering all induction impossible. This lamentable state of things is more particularly striking in the case of syenites, and the purpose of this paper is to call attention to the difference between the English and American schools, as represented by Lyell, Dana, and Jukes, and the German schools, as represented by Bischof, Werner, Cotta, etc.;—the latter using the term syenite to express a distinctly plutonic basic rock, poor in magnesia, and closely allied to diorite and its congeners; the former using it to express an acidic rock, rich in magnesia, and allied to granite, gneiss, and schist; thus the one points to the probable presence of tin, copper, lead, and zinc, while the other only points to antimony and zinc. Gold and silver are common to both.
In all cases this difference is highly objectionable, but especially so here in New Zealand, where so little is known of the true structure of the country, and many parts are seldom visited by white men, it is of the highest importance that every one should call the same thing by the same name.
On the table are specimens of syenites of the different schools, and it will be at once apparent how much they differ; it will also be observed how nearly the one approaches a granite, and the other a diorite.
To compare the definitions of Lyell, Dana, and others;—
Lyell and Dana. Syenite resembles a granite in which the mica is replaced by hornblende, also that the felspar may be orthoclase or oligoclase; this is, therefore, eminently acidic, but the acidic character is not constant or characteristic.
Cotta, on the other hand, says that syenite consists of orthoclase or microlene and hornblende, which may have mica and quartz as accessories, but if either of these are abundant it at once becomes syenitic granite or syenitic gneiss—the difference is more apparent by comparison with diorite and granite.
Dana:—Felspar, quartz, hornblende.
Dana:—Felspar, quartz, mica.
Cotta:—Felspar, quartz, mica.
Dana:—Felspar and hornblende Triclinic.
Cotta:—Felspar and hornblende Triclinic.
From the above it will be at once apparent that the German classification is not practical, i. e., it cannot be used in the field, for it is rather too much to expect that a geologist, with only an acid bottle, pocket lens, and knife, can decide with any degree of certainty on the angles a half-embedded crystal makes, or the system it belongs to; for although orthoclase is monoclinic and oligoclase triclinic, still the angles are very nearly the same, (albite) 118°a or 120°o 86°a or 90°o etc., etc. The inconvenience of this classification is well shown by the fact that Werner, who first proposed it to suit a certain rock, subsequently called the same rock a diorite. If, however, the English system is adopted, there can be no hesitation in at once determining the quartz, felspar, and hornblende; felspar and hornblende come in as hornblende rock if the felspar is decidedly triclinic, if not, as a diorite.
The English school, or rather the American branch of it, has many claims to preference over all others, chiefly on account of its simplicity, and this simplicity is insured for some time to come, as the Americans have so much room for real practical science, and cannot afford to waste time and talents in multiplying names and then finding out some compound to suit them; the old proverb about a certain personage finding mischief for idle hands, applies to the natural sciences as well as anything else, for when a certain point has been
reached, many men seem to get tired of good healthy work, and instead of exploring fresh fields go over the old, and keep magnifying minor differences into groups of families, to the complete confusion of everything; forgetting that the great aim of all science is simplicity, and the more simple a science the grander and nobler it is.