586 8. Allport — Nomenclature of Rocks. 



have recourse to hypothetical reasoning on the subject, for there is 

 the clearest evidence that the so-called plutonic rocks are in reality 

 the lower portions of true lava-streams which have reached and 

 flowed over the surface. In that most interesting and important 

 contribution to science which Mr. Judd recently laid before the 

 Geological Society (Quart. Journ. vol. xxx.), he has clearly shown 

 that, among the Tertiary volcanic rocks of the Hebrides, deeply 

 formed granite passes by insensible gradations into felsites which 

 have reached the surface ; and that the coarsely crystalline gabbros 

 of the central parts of the volcanos also pass gradually iuto dolerites 

 and basalts, which formed lava-streams of enormous extent. 



Admitting, then, the inaccuracy of the prevailing views on the two 

 points just mentioned, the question of classification and nomenclature 

 becomes considerably narrowed and simplified. For if it be the 

 fact that there have been true volcanos from the earliest period in 

 which there is evidence of life, or of the earth being in a habitable 

 condition, and also that the products of volcanic action have been, 

 with perhaps a few exceptions, the same from the earliest to the 

 present period, it becomes evident that they should be regarded as 

 forming one natural group, and classified in accordance with such 

 definite and ascertained characters as they may be found to possess. 

 Some of these have been long recognized. The separation, for ex- 

 ample, of the entire series of volcanic rocks into the two classes of 

 basic and acidic, is simply the expression of a well-ascertained fact, 

 which may be verified in every quarter of the globe ; and the sub- 

 division of the latter into rocks possessing a granitic, felsitic, and 

 vitreous texture, with their respective porphyritic varieties, although 

 based on more special characters, is nevertheless of wide application. 



So far, there are but few difficulties to contend with ; but in the 

 adoption of specific names, there is abundant opportunity for those 

 who like to exercise their ingenuity in the discovery of a new species 

 in every variety of rock that happens to fall into their hands. They 

 havea lready made the list too long, and it might be at once 

 advantageously curtailed by the rejection of several synonymous or 

 otherwise, bad terms. Among these I should include : felstone, fel- 

 spar porphyry, quartz-porphyry, and elvanite, which would be better 

 named : felsite, porphyritic felsite, and porphyritic quartz-felsite, 

 though the latter is rather long. Elvanite is, however, a bad name 

 for it, as the rock is not, as was supposed, really characteristic of the 

 Cornish elvans. An examination of a large series of these rocks 

 clearly shows that they are simply dykes, very many of which con- 

 sist of typical granite, while others are quartz-felsites which are 

 frequently porphyritic in texture. 



In whatever sense the local Cornish term elvan may have been 

 originally used, it has now no very definite application, for in 

 addition to the granitic and felsitic elvans, there are the ' blue ' and 

 ' grey ' elvans of the miners, some of which are altered gabbros and 

 dolerites, or even hard altered slates. 



In conclusion, it would, I think, be premature at present to suggest 

 any great changes either in classification or nomenclature ; but from 



