584 S. Allport — Nomenclature of Rocks. 



variety of liparite,' a ' granitic rhyolite,' an elvanite, or a siliceous 

 elvanite, for the definition given would be quite as applicable to any 

 one as to the others. Nevadite is then said to represent " the 

 passage rock between trachyte and normal granite ; similarly, as a 

 siliceous elvanite among the older rocks, is the passage rock between 

 felstone and normal granite." The author then informs us that he 

 suspects the existence of passage rocks between augite and granite ; 

 and these are subsequently mentioned as "maculated basic rocks 

 which seem to graduate into dolerite and augite." It has, of course, 

 been long known that the extremes of the acidic and basic series 

 overlap, and that some of these intermediate forms exhibit a complete 

 transition from the trachytes to the dolerites ; they are, in fact, the 

 trachy-dolerites of Abich, and there are rocks of quite similar com- 

 position belonging to the older geological periods ; but a rock inter- 

 mediate between augite and granite must be something new, and 

 petrologists will, no doubt, be glad to learn something more about it. 



If to the ten names above mentioned be added andesite, dacite, 

 and domite among the more recent rocks, and felsite, petrosilex, 

 felspar porphyry, quartz porphyry, hornstone porphyry, eurite, 

 pegmatite, granitite, etc., among the older series, the reader will have 

 some faint idea of the amount of confusion introduced into the 

 nomenclature of one group of rocks by the mis-directed ingenuity of 

 those who, like species-makers in other branches of natural science, 

 are never so well pleased as when inventing new names for mere 

 local varieties. 



Now it appears to me, that if we wish to introduce something like 

 order and simplicity into our rock-nomenclature, we may as well 

 commence by discarding the old notion of an essential original dif- 

 ference between volcanic rocks of different geological periods ; and 

 I imagine there will be found a general disposition among geologists, 

 not only to deprecate the introduction of new names for mere 

 varieties, but also to insist on the necessity of reducing the number 

 of those now in common use. The existence of the two great groups 

 of acidic and basic rocks has long been recognized, not only among 

 the products of recent volcanos, but also in association with strata 

 belonging to the older formations. I have shown elsewhere, that 

 basic rocks of widely separated geological periods are identical 

 in composition and structure, and every additional investigation 

 clearly indicates that the same generalization may be applied to the 

 members of the acidic group. In the older series, for example, there 

 are the felsites or felstones, and the porphyritic felsites or " felspar 

 porphyries," which with the addition of free crystallized quartz, 

 become " quartz porphyries," " elvanites," etc. ; and among the 

 Tertiary or recent rocks there is the strictly corresponding series of 

 trachytes, porphyritic trachytes, quartz trachytes or liparites. The 

 mode of occurrence of all these rocks is precisely the same, and the 

 members of the two groups cannot be distinguished from each other, 

 either by mineralogical composition or structure. 



It will probably not now be disputed, that there are true granites 

 of all ages, and sooner or later it will be recognized, that the old 



