476 Correspondence—Leonard H. avwkes. 
foregoing underwent extensive denudation. (4) Huronian system, 
including the Bruce and Temiskamian series. (5) Algoman revolution: 
the intrusion of the Algoman granites. (6) The Eparchzan interval 
of denudation. (7) This was followed by the deposition of. the 
Animikian and Keweenawan series, forming the Algonkian system, 
which the author prefers to regard as being the base of the Paleozoic. 
This concise statement will do much to simplify and render clear 
a subject in which much confusion has hitherto prevailed, largely 
owing to an unnecessarily cumbrous nomenclature, but partly also 
due to uncertainty in the correlations. 
VI.—Mouscuerratk IcurHyosaurs.—Von Huene’s manuscript. on 
the Muschelkalk Ichthyosaurs, dated autumn, 1913, has just appeared 
in Pale@ontographica, vol. lxxii, pt. i, June, 1916. The work consists. 
of 68 pages, 7 plates, and 96 text-figures, and is of the greatest 
use for comparison and study. Von Huene describes the genera 
Mixosaurus, Cymbospondylus, Shastasaurus, Pessosaurus, and a new 
genus Pachygonosaurus, founded on a few vertebre in the Berlin 
Museum, indicative of two species to which no trivial names are 
assigned. ‘he illustrations are abundant and good. 
CORREBSPON DEHN CEH. 
. ROPY SURFACES OF LAVA IN ICELAND. 
Srr,—In last month’s Grotocican Macazine, writing of the ropy 
lava surfaces so characteristic of large areas of the Icelandic deserts, 
I remarked on never having seen similar surfaces in the Tertiary 
series of Iceland, and added that they do not seem to have been noted 
in the British Isles. 
I find, however, that basalts with ropy surfaces were observed in 
the Ferde Islands by Sir George Mackenzie and described by him 
to the Edinburgh Royal Society in1812. Mackenzie’s paper (‘l'rans. 
Roy. Soc. Edin., vol. vii, p. 218, 1814) contains a natural size 
illustration of ropy basalt. He writes: ‘‘The surfaces of many 
lavas which I passed over in Iceland were not unlike coils of rope or 
crumpled cloth, an appearance which we should expect to be assumed 
by any viscid matter in motion., On our first visit to the island of 
Naalsde we observed the surface of a bed of amygdaloid, which had 
been exposed to a considerable extent by the removal of the bed 
above, exhibiting an exact picture of the lavas I had seen in Iceland” 
(p. 221). Mackenzie states that whatever doubt may have previously 
existed in his mind as to the igneous origin of the ‘trap’ was 
dissipated by the discovery of these ropy surfaces.! 
Leonard HawkEs. 
GLASGOW. 
‘ In reference to the footnote to my paper, GEOL. MAG., Sept. No., p. 390, 
I find that Dr. Pjetursson now regards his ‘‘ Graa Etage’’ of Middle Northern 
Iceland as of Post-Tertiary age. This view presents difficulties, and I prefer 
to hold to his former idea as there indicated. ‘The doubt I expressed relates, 
not to the Tertiary age of the ‘‘Graa Htage’’, but to the adequacy of the 
evidences therein of a general ‘‘ glacial period’’. 
