SOME NORTH-OF-ENGLAND DYKES. 297 
Many of the so-called augite-andesites contain a rhombic pyro- - 
xene; and it has been suggested by Mr. Whitman Cross (‘ Amer. 
Journ. of Science,’ Feb. 1883), in a paper on the andesite of Buffalo 
Peaks, Colorado, that this rhombic pyroxene (hypersthene) is the 
predominating bisilicate in these rocks*. If this should turn out to 
be correct then they would have to be termed hypersthene-andesites, 
and the question would arise as to whether the term augite-andesite 
should be retained at all. I think in any case, however, the term 
augite-andesite is useful to indicate such a rock as the one described 
above, that is a plagioclase-augite-magnetite rock, having more or less 
a trachytic or andesitic structure, a silica percentage considerably 
higher than that of normal basalts, and a specific gravity of about 2-7. 
At the same time it must be admitted that such a rock is not sepa- 
rated from basalt by any hard-and-fast line; and from this point 
of view it might be called an andesitic basalt, except that this term 
has been used in a somewhat different sense by Boricky (‘ Basalt- 
gesteine Bohmens,’ p. 543). 
A rock having essentially the same chemical and mineralogical 
composition has been described by Prof. A. Geikie, in a paper on the 
so-called Pitchstone of Eskdale (‘ Proceedings of the Royal Physical 
Society of Edinburgh,’ vol. v. 1880). The Pitchstone of Jameson is 
shown to be merely a glassy form of this rock. My attention was 
called to this paper by Mr. Topley, after the above description of the 
microscopic character of the Cockfield and Armathwaite rock had 
been written. I mention this because there is a very striking resem- 
blance between the way in which Prof. Geikie has described the 
devitrification-products of the so-called pitchstone and the manner 
in which I have described the precisely similar bodies in the 
Armathwaite specimen. ‘The figure which Prof. Geikie gives in the 
paper above quoted, and also in his ‘ Text-book of Geology,’ p. 100, 
exactly represents the special features of the base of the Arma- 
’ thwaite specimen (compare Pl. XII. fig. 2 a). 
It may be as well to quote the bulk-analysis of the Eskdale 
rock :— 
Siloam lte lr, resi tren ua Nags 58°67 
JMG STTH OTE Hoe nee ke Mane en tire 14°37 
HEREOUSKOXIGCy We) games cee 6°94 
Nea Chay Be oe ecu 1:64 
Manganese oxide ........ trace 
Whine nue wseney a retadte ee ode ane 7°39 
IVI OO STi ee eye eta) saat eo. 4°65 
RO GAS ey HNe. eiee cca. ausa Sie. 14 
SOG: Ueree eae Seals aloneaiee 3°01 
AINE Ge eae St Sega I epee gener 2°02 
100°11 
* See also Bulletin of the U.S. Survey, no. 1. 
Q2 
