226 J.J. H. TEALL—-PETROLOGICAL NOTES ON 
General Relations of the Rock forming the Cockfield and Arma- 
thwaite Dyke. 
The rock has generally been considered a basalt; but it differs 
from typical basalts in many ways: thus olivine is entirely absent, 
and the bisilicate constituent plays a very subordinate part in the 
mineralogical composition of the rock. Then, again, the silica per- 
centage (57 to 59) is much too high, that of true basalts averaging 
about 50 or less, and the specific gravity (2°77) is too low. The 
presence in considerable quantity of a highly acid base is also 
another point which distinguishes this rock from true basalts. Now 
in all these points the rock approaches the andesites and trachytes ; 
and if the term augite-andesite is to be used at all, it seems fairly 
applicable to this rock, which, be it remembered, is almost certainly 
of Tertiary age. 
IT append a Table of analyses of recognized augite-andesites to 
show that there is a close correspondence between them and the 
rock now under special consideration :— 
i dite III. TV: oo 
Sica eee ee 57°80 | 59-70 | 5876 | 57-51 | 56-582 
Jini 18:07 | 1616 | 17:34 | 1683 , 18379 
Frecly ORS on. suesenceen 8-98 797 | 777 | 1040 | 1-112 
Nervous OxXtdGr. veo. ecesccl scecene 0°83.) 3 ee | 5-995 
Manganese Oxide, ......20-|) 4occcag f,  --csco 4 |) eee — 07108 
Tova ceee ee 4-69 8-01 7-46 654 11-053 
Magnesia ..dtvciecss.ctees 1:12 116 2-67 1:89 3-239 
Poet a) eee a | 261 1:56 0-93 221 0-912 
Ge are Wee ae 4:60 | 3-12 2:36 3:36 2589 
Lisa: ee eee SNS 1-18 1:09 2:10 0-43°| a8 
99:05 | 99:60 | 99:39 | 99:67 , 99-969 
I. Augite-andesite Lava, Polsiegy near Szkaros.—Dr. C. Dolter, “ Ueber einige 
Trachyte des Tokaj-Eperieser Gebirges,” Min. Mitth. 1874, Heft iii. p. 213. 
II. Augite-andesite from Kohegy near Bogdanz Garbocz.—Dr. C. Dolter, Min. 
Mitth. 1874, Heft ui. p. 207. 
III. Porphyritic Augite-andesite from Tuhrina.—Dr. OC. Dolter, Min. Mitth. 
1874, Heft iii. p. 205. 
IV. Lava of Pariou, ending at Fontmort, Clermont Ferrand.—Von Lasaulx, 
“Petrographische Studien an den vulkanischen Gesteinen der Auvergne,” Neues 
Jahrbuch, 1871, p. 682. 
V. Lava of Masaya Mindiri, Central America.—H. R. Marx, “ Beitrag zur 
Kenntniss centralamerikanischer Laven,” Zeitschr. d. geol. Ges. xx. p. 526 (1868). 
The bulk analysis of our rock also bears a close resemblance to 
that of many of the so-called melaphyres, as will be seen at once by 
referring to the collection of analyses published in Zirkel’s ‘ Lehr- 
buch der Petrographie,’ vol. ii. p. 55. It should be borne in mind 
that the rocks to which the term melaphyre has been applied on the 
continent have, as a rule, a higher silica percentage, a lower specific 
eravity, and a larger proportion of felspar than the basalts. | 
The affinities of melaphyre are therefore rather with the augite- 
andesites than with the true basalts. 
