^()Q THE CANADIAN NATURALIST. [Vol. vl. 



In preparing tabic III, tlic snmo cnrc has been taken as with 

 those already given to introduc3 no new terms, and to use the 

 "arious n;ini2s of the species only in the sense which at present is 

 generally attached to them by petrologists. In a few instances, 

 where such n.imos have hitherto borne a too general or a more or 

 less indefinite meaning:, an attempt has been made to confine their 

 applic ition to one species. The name rhyolite is for instance used 

 in a somewhat more restricted sense than that given it by its 

 originator, and the very vague, generally condemned, but still 

 much used or misused, name, melaphyrc, is, as applied to a parti- 

 cular species, limited to those porphyrite rocks which are neutral 

 in chcmicil composition and in which crystals of triclinic felspars 

 only are developed. In some other cases, where the same species 

 possessed several synonyms, a slightly different signification has 

 been given to one, and generally the least used of them, in order 

 to make it of use in our .system. For instance, curite and felsite 

 have hitherto been synonymous. In our table the latter term is 

 made to indie itc the more silicic species of fine grained rocks. 

 Such names of rocks as have been derived from those of minerals 

 have their terminations, in accordance with Dana's sucrirestion, 

 altered from ife to tjte. 



It will be observed that, in table III, the minerals of the fels- 

 pathic class only are placed at the head of the vertical columns, 

 while the other essential minerals have been placed under each 

 variety of texture on the left hand side. The cause of this 

 arrangement may here be stated. The felspars, being of very 

 constant occurrence in original rocks, and being frequently diffi- 

 cult to determine, have not been much made use of in distin- 

 guishing species until quite recently. For instance, oligoclase 

 very often can only be distinguished from orthoclase by an 

 experienced mineralogist, and only an experienced chemist 

 after a minute analysis, can distinguish between oligoclase, 

 labradorite and anorthite in a compound rock. On the other 

 hand the minerals of the other class possess very well marked 

 physical characters, and the presence of one or other of them 

 was readily detected by the earlier petrologists and made 

 use of by them for characterising different rocks. Thus, mica, 

 hornblende and olivine are very widely apart both as regards 

 form, colour, hardness and fusibility. The only two minerals of 

 the second and third classes which are difficult to distinguish 

 from each other are hornblende and augite, and this is only the 



