232 Reviens — Br. J. Petersen on Cheviot Bodes. 



relative age of the rocks under his consideration ; otherwise petrology 

 "would speedily cease to be one of the most important branches of 

 geological teaching, which it at present undoubtedly is. That a 

 difference in age should, to a certain degree, find expression in 

 nomenclature, I hold to be quite justifiable ; for it is in almost every 

 case accompanied by a great difference of general habit, and in part 

 even of mineralogical constitution — a case in point is for instance 

 the restriction of hauyne and leucite to the Tertiary period. 



" It cannot be denied that it is frequently very difficult to decide 

 upon the age of eruptive rocks, and that occasionally it may be 

 impossible to do so ; yet the present status of petrology enables us, 

 by comparison with other well-known rocks, in most cases to arrive 

 at a very probable — if not a perfectly precise — determination of the 

 age. 



" Eeally doubtful cases, whether a rock be Pre- or Post- Tertiary 

 will be very rare, and for such we do not require a new classification 

 and nomenclature which leaves geological age out of consideration." 



The first point which requires notice in the above quotation is the 

 one raised with reference to the meaning which is to be attached to 

 the term porphyrite. In the Cheviot district we have an immense 

 development of rocks to which every one would apply the term 

 porphyrite. These are all of them more or less altered. Indeed 

 it is their alteration which gives them many of their distinctive 

 characters. These porphyrites are sharply to be distinguished from 

 the rock which I have called hypersthene-andesite, although 

 some of them are undoubtedly due to the alteration of a rock 

 of similar mineralogical composition. Inasmuch as these altered 

 rocks have a distinct character of their own, I see no objection 

 to giving them a distinct name. The author seems to consider 

 that under no circumstances should pathological characters be used 

 for purposes of petrological classification. On this point I cannot 

 agree with him. If the altered rocks have a definite character, 

 in virtue of their alteration, and are widely distributed in space, 

 I see no reason why they should not receive a distinct name. 

 The principle is well established in geological nomenclature, as, 

 for instance, in the use of the term serpentine. 



I may say, en passant, that I have seen among porphyrites rocks 

 that I should describe as altered augite- hypersthene- hornblende- 

 and mica-andesites. 



The next point referred to is the use of the term andesite. 

 It is with very great pleasure that I see the author admits the 

 resemblance between the unaltered rock and modern andesites. 

 No attempt is made to show that there is any essential distinction 

 either in structure or composition. This reduces the difference 

 between us to the simple question — should geological age in itself 

 be made a point of classificatory value in petrology ? On this point I 

 feel compelled to differ from the author. 



He says, " Just as it is the problem of the palaeontologist to 

 determine the age of sedimentary deposits, so is it an important 

 question for the petrologist to determine the relative age of the 



