Hevieics — Dr. J. Petersen on Cheviot Rocks. 231 



On comparing this with the analysis of the base of the unaltered- 

 rock, it is seen that the pei"-centage of water is much less. The 

 specific gravity of the altered base is higher. 



The author then gives analyses of the normal porphyrites. 



I. II. III. IV. 



SiOs 63-0 61-17 64-2 59-0.5 



AI263 14-9 16-87 16-0 15-69 



FeaOg 4-7 2-10 4-3 1-80 



• FeO — 2-94 — 4-72 



CaO ... .. 4-8 4-86 1-7 1-79 



Ms^O 2-8 3-00 2-5 4-29 



KoO 1-9 1-81 5-9 2-88 



NaoO 4-0 2-67 2-9 3-97 



H,0 4-0 3-09 3-3 3-16 



100-1 98-51 100-8 97-45 



Sp.gr 2-54 2-543 2-56 



I. Coquetim. above Windy Haugli(T. Waller, Geol. Mag. Dec. II. Vol. X. 108) : 

 II. Carhope on Coquet (Petersen). 



III. I m. above Shillmoor Farm, Coquet (T. Waller, Geol. Mag. Dec. II. X. 151). 

 IV. ^m.. above Shillmoor Farm, Coquet (Jager). 



In discussing analysis III. the author is under the impression that 

 I regard the rock as derived from the hypersthene-andesite by 

 alteration. This is not the case. He has entirely misimderstood 

 the sentence in which I refer to this analysis, and I agree with him 

 that the opinions he attributes to me are inconceivable. 



We now have to consider the question of nomenclature, and as 

 this is a point of considerable importance, it will be as will to allow 

 the author to speak for himself, as far as this is possible in a trans- 

 lation : — 



" I take the liberty of adding a few words to justify the designation 

 of the described rock as enstatite-porphyrite. When the rock is fresh, 

 Mr. Teall calls it an andesite ; when it is altered, a porphyrite. In my 

 opinion it is inadmissible to give a different name to different hand- 

 specimens obtained from the same rock-mass according to the state 

 of freshness of the components. Mr. Teall himself acknowledges 

 that the porphyrites are in the main only altered andesites. In his 

 first paper he defines the andesite more particularly as an augite- 

 andesite ; in his paper in the April Number of the Geol. Mag. he 

 introduces the term hypersthene-andesite. In defence of the term 

 andesite he points out that the rock has the closest resemblance to 

 the Santorin lavas, characterized as andesites by Fouque, This 

 resemblance certainly exists to a large extent. The name " andesite " 

 has, however, so far only been applied to younger Tertiary or Post- 

 Tertiary rocks. Mr. Teall makes use of this name for a rock which 

 he spealvs of as in all probability belonging to the Old Eed Sandstone 

 period. In justification of this, the argument is advanced that a 

 petrologist ought to be in a position to define a rock regardless of 

 its age, as it is frequently difficult to determine the latter. 



" In my opinion this is unnecessary. 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 



