106 J. J. Harris Teall—Cheviot Andesites and Porphyrites. 
felspars form a large portion of the entire mass of the rock, and the 
felspars of the ground-mass vary in size from those giving lath- 
shaped sections (‘4 mm. by -14 mm.) to very small microlites, fluidal 
structure is absent; in the other and more interesting type the 
porphyritic felspars are not so abundant, the felspars of the ground- 
mass appear as innumerable, minute microlites, and the whole section 
shows the most exquisite fluidal structure. It is this type which so 
closely resembles the Santorin lava of 1866. I have observed it only 
in some of the Alwin boulders. 
GENERAL RELATIONS OF THE ROCK. 
There exists, as is well known, between the basic and acidic 
igneous rocks, an extensively developed and widely distributed inter- 
mediate group. The trachy-dolerites of Abich,’ and the andesites 
of Leopold von Buch,? belong to this group. Of these two terms 
the former has almost died out of petrological literature, while the 
latter has become definitively established, owing to the work of 
Groth, Zirkel, Rosenbusch, and others. An andesite, in the widest 
sense of the term, may be defined as a plagioclase-pyroxene (using 
the term pyroxene to include the entire group of the bisilicates) rock 
with a specific gravity of from 2°55 to 2:9, and a silica per-centage 
lying between 55 and 68. The entire group has been divided by 
Groth into hornblende-andesites and augite-andesites, and this di- 
vision has now become perfectly well established. The group as a 
whole shades off through the hornblende-andesites into the trachytes, 
and through certain of the augite-andesites into the basalts. 
The idea of hornblende-andesite has become fairly well fixed, but 
that of augite-andesite is still vague and fluctuating. The most 
precise definition of this, as of many other petrological terms, is 
contained in Prof. Rosenbusch’s critical work on ‘The Microscopic 
Physiography of Massive Rocks” ;* a work for which all petrologists 
owe the author a deep debt of gratitude. One gathers from the 
description there given that the typical augite-andesites are porphyritic 
rocks, containing two generations of felspars, together with augite, 
magnetite, and a base which may be either glassy, micro-felsitic, 
erypto- or micro-crystalline. Quartz, hornblende, biotite, hematite, 
and ilmenite occur as accessory constituents in different varieties. 
The author, however, does not lay stress on the porphyritic character 
of the rock, and we find Dr. Hugo Biicking,* who claims to use the 
term augite-andesite in the sense adopted by Prof. Rosenbusch, apply- 
ing it to a rock which occurs in the Rh6n district, and which had 
previously always been regarded as a dolerite. Prof. Sandberger ° 
objects, and I think with justice, to this extension of the term, and 
points out that the rocks in question are essentially similar to the 
typical dolerite of Meissner originally described by Hany. 
1 Abich, A., Ueber die Natur und die Zusammenhang der vulkanischen Bildungen. 
Brunswick, 1841. 2 Humboldt’s Kosmos. 
* Mik. Phy. d. massigen Gesteine, p. 407. 
4 Mineralogische Mittheilungen, 1878, vol. i. p. 588. 
> Min. Mitt. 1878, p. 280. 
