148 J.J. Harris Teall—Cheviot Andesites and Porphyrites. 
dominating bisilicate, if we may judge from a single microscopic 
section, a twinned monoclinic and slightly pleochroic pyroxene. 
This rock looks then as if it might be a true augite-andesite. 
It thus appears that many of the rocks hitherto known as augite- 
andesites ought rather to be called hypersthene-andesites, and the 
question arises what rock are we to regard as typical augite-andesite ? 
In connexion with this, I should like to point out, as indeed is well 
known, that there are undoubtedly non-olivine bearing plagioclase- 
augite rocks of a porphyritic character, with a silica per-centage 
varying from 55 to 59, and a specific gravity of from 2:7 to 2:8; such, 
for instance, as the rock forming the Cleveland dyke, which is, be it 
remembered, certainly Post-Jurassic, and almost certainly Miocene. 
These can scarcely be called basalts, and it becomes a question 
therefore whether the term augite-andesite may not with propriety 
be applied to them. If so, we should then have as the generic 
term pyrowene-andesite, and as specific terms, hornblende-andesite, 
hypersthene-andesite, and augite-andesite. In a short time I hope to 
publish a detailed description of the rock of the Cleveland dyke, and 
it can then be decided whether such a rock has any claim to the now 
somewhat discredited term augite-andesite. 
Microscopic ANALYSIS OF THE CHEVIOT PORPHYRITES. 
These rocks are for the most part only altered andesites, and it 
becomes a question, therefore, whether the term porphyrite should 
be applied to them at all. There is no doubt that in this country we 
have included under the general term porphyrite rocks of extremely 
diverse character ; and a considerable difficulty therefore arises when 
we attempt to give the term a precise petrological signification. 
Considerable work will have to be done on the porphyrites before it 
becomes advisable to remodel the terminology; meanwhile, it is. 
important to note that a large number of the Cheviot porphyrites are 
merely altered andesites, and therefore quite different from the 
felspar-magnetite rocks, which are described as porphyrites by Prof. 
Geikie in his paper on the Carboniferous Volcanic Rocks of: the 
Firth of Forth." 
For the purpose of description the Cheviot porphyrites not belong- — 
ing to the type described in my previous paper, in which the dis- 
tinguishing macroscopic feature is a resinous lustre, may be divided 
into two groups according to the presence or absence of mica, and it 
is worthy of note that Szabd divides the Hungarian trachytes, under 
which term he includes the andesites, in a similar way. Whether 
the mica porphyrites of the Cheviots occur as true lavas, I am not 
able to state definitely ; but they certainly occur as dykes, and in this 
respect, as well as in others, they are related to the mica-bearing 
quartz porphyries or porphyrites which occur in the same district. 
To these latter rocks, however, I do not propose to refer in the 
present paper. The porphyrites without mica are by far the most 
1 Trans, Royal Soc. Edin., vol. xxix. part 1. 
