8. Allport — Nomenclature of Rocks. 585 



felstones and porphyries were originally identical with the more 

 recent trachytes ; corresponding varieties occur in both series, and 

 microscopic examination clearly shows that the difference observable 

 in some of the older rocks is the result of chemical or other meta- 

 morphic action to which they have been exposed. 



There is, however, an occasional difference in texture which should 

 not be overlooked. There can be but little doubt that in the intru- 

 sive sheets of various old rocks we have before us some of the pro- 

 ducts of volcanic action which have been formed far below the 

 surface, or, at any rate, beneath great piles of ejected and loosely- 

 aggregated materials, which have been subsequently removed by 

 denudation. Bocks thus formed under pressure might be expected 

 to differ considerably in structure, if not in composition, from those 

 poured out on the surface ; and this is frequently, though by no 

 means invariably, the case : for the central parts of many lava flows 

 are as compact, and exhibit precisely the same texture, as the sheets 

 which have been intruded among the surrounding strata. Generally, 

 however, the upper and lower surfaces of true lava-flows are dis- 

 tinctly vesicular or scoriaceous, a character not exhibited by intru- 

 sive sheets, but one quite as common in Silurian or Carboniferous 

 lavas as in those of recent formation. 



Intimately connected with this subject is the old distinction be- 

 tween the so-called plutonic and volcanic rocks, a distinction which 

 I have long held to be entirely erroneous in the sense in which it is 

 frequently employed, but which is still maintained by authors of the 

 highest repute, more especially among our German friends. As an 

 example, I may adduce the last edition of Naumann's "Lehrbuch der 

 Geognosie" (vol. ii. p. 63), in which it is made the basis of his 

 classification, the plutonic formations being characterized as eruptive 

 rocks not formed by true volcanos. A list of the rocks is then 

 given, and among them are included diorite, diabase, augitporphyr, 

 gabbro, hypersthenite, melaphyr, with their conglomerates and tuffs. 

 But all these rocks are now well known to be true volcanic products ; 

 the so-called melaphyres, diabases, etc., of Silurian and Carboniferous 

 ages are frequently found regularly interstratified with beds of ash ; 

 the separate flows are scoriaceous and slaggy at top and bottom, 

 and they are evidently as true lavas as any of those ejected by still 

 active volcanos. 



It appears, then, that there are two fallacies underlying 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 constitution. In the first place it may be remarked that 

 the doctrine laid down in the former proposition can be nothing 

 more than a mere assumption, which, from the very nature of the 

 case, can seldom or never be capable of demonstration; for if certain 

 masses were originally formed far below the surface, and are now 

 exposed above it, the overlying rocks must have been removed, and 

 with them has disappeared all evidence as to their volcanic or other 

 mode of origin. Fortunately, however, it is not now necessary to 



