G. II . Kinahan — Classification of Rocks. 115 



selected from persons eninent in these different pursuits. The classi- 

 fication proposed by Mr. Allport might do for Lithologists ; but it 

 would not do for Petrologists, as the latter find in the field that 

 siliceous rocks graduate into basic ; also that nearly invariably some 

 portions of a dyke or felsite will be porphyritic ; while it is rare to 

 find a felsite that is not quartzose ; in some the quartz being more 

 conspicuous than in others.^ 



Mr. Allport states that he has " shown elsewhere that basic rocks 

 of widely separate geological periods are identical in composition 

 and structure." This may be, to his own satisfaction, but not to 

 that of all chemists and microscopists ; as Forbes and others seem to 

 be of the opinion that this statement is not proven, and that the 

 basic rocks of the widely separate geological periods contain different 

 felspars. For my own part, in the field, I find that, after a perfect 

 knowledge of the rocks is acquired, it is generally possible to say 

 to what geological age the different rocks belong. This observer 

 also gives what he considers two fallacies imderlying the present 

 system of classification: — ■" 1st, that plutonic rocks have not been 

 formed in connexion with true volcanos ; and, 2nd, that rocks of 

 different geological ages are characterized by a difference in mineral 

 constitutions." Of the second it is only necessary to point out, that 

 up to the present time, it does not seem to have been proved to be a 

 fallacj^ The first is a fallacy in a certain sense, in others it is not. 

 The terms Volcanic and Plutonic have been used loosely, similarly 

 to all other rock names. Strictly speaking, volcanic rocks are those 

 formed at the surface, while plutonic rocks were formed below the 

 surface, and the granitic rocks at still greater depths ; therefore all 

 volcanic rocks must have been formed in connexion with plutonic 

 and granitic rocks. But it does not necessarily follow that all 

 granitic and plutouic rocks must have been formed in connexion 

 with volcanic rocks ; and many of the older igneous rocks were 

 irrupted under quite different circumstances from those under which 

 volcanic rocks were and are erupted. 



As pointed out by Le Conte and others, a volcano is a mere flyblow 

 on the earth's surface, due to the dying out efforts of Vulcanicity, 

 and made up of scoria and the like, traversed by dykes and associated 

 with lava-streams that sometimes form small sheets. The older 

 igneous rocks sometimes seem to be the remains of ancient volcanos ; 

 but more generally they are the products of great fissure irrup- 

 tions, which ejected vast quantities of molten and other matter, 

 which now form extensive sheets. This is exemplified by the Irish 

 rocks. In the province of Ulster during the Miocene (?) period, 

 there were large successive outpourings of igneous rocks, that 

 covered miles of country with sheet after sheet of rocks ; while 

 subsequently the vulcanicity seems to have died out in volcanos at 

 isolated localities. — In the Carboniferous Limestone of the central 

 plain (Leinster and Munster) there seem to be the remains of vol- 



^ Mr. Allport objects to the term elvanite because it has been misused. On the 

 some principle nearly all the rock names ought to be rejected ; especially granite, as 

 the latter is a common name in many places for hard and coarse sandstones. 



