886 Leonard Hawkes—Building up the North Atlantic 
of red rock, which gives an average thickness of 26 feet for each 
flow and its accompanying parting. Two flows were 55 feet thick, 
one was 6 feet, and one was seen to thin out altogether. The basalts 
preserve a constant thickness for great distances, but when they thin 
out they do so rapidly. The interbasaltic layers are composed of 
a soft colloidal rock a few feet in thickness (commonly two or three 
feet) and of the variable chemical composition characteristic of the 
members of the laterite group of rocks. In the present discussion 
these beds are referred to as ‘ red partings’ to distinguish them from 
the more varied deposits occurring at rarer horizons. 
The prevalent view with regard to these red partings has been 
expressed by Sir A. Geikie, writing on the Antrim plateau: “It” 
(i.e. the red interbasaltic layer) ‘‘may be looked upon as probably 
furnishing evidence of the lapse of an interval sufficiently extended 
to permit a considerable subaerial decay of the surface of a lava-sheet 
before the outflow of the next lava” (1, p. 204). Some, on the 
other hand, advocate the original tuff nature of the red rock, and, as — 
is pointed out below, this question is of some importance when we 
attempt to read the history of the Tertiary volcanic period. 
Thoroddsen writes of the Icelandic series: ‘‘ Der Aufbau des 
grossen Plateaus hat ungeheure Zeit beansprucht. Dass die 
Lavastrome nicht unaufhaltsam tbereinander geflossen sind, ist 
unter anderen aus den zwischen den Basaltdecken befindlichen 
Verwitterungsprodukten ersichtlich’’! (8, p 232). In the latter 
part of this paper an attempt is made to indicate more definitely the 
solution of this problem. 
Tue Rep Parrines. 
As a rule it is only by microscopical examination that the original 
tuff or lava nature of the red rock can be determined. | Its colloidal 
character makes the preparation of thin slices difficult, though good 
results can be obtained by using petroleum as a lubricant instead of 
water. Professor Amund Helland has described thin sections of 
similar Icelandic rocks, but his paper, written in Norwegian, 
has escaped notice in Britain (4). Several sections were made of 
specimens taken at random from the Tertiary series. That shown in 
Pl. XVI, Fig. 1, is of a less weathered rock than usual, and the nature 
of the parent rock is clearly indicated. A complete petrographic 
description is not given here, as it is only desired to draw 
attention to one important point in connexion with the process of 
palagonitization. 
The bulk of the material shown in the section is volcanic glass; 
one large piece of scoria is seen at ‘A’. The glass fragments have 
characteristic smooth-flowing’ outlines, those of the scoria being 
irregular and hackly. The glass is basic, the freshest being light 
yellow in colour and forming the central parts of the fragments, 
1‘ The building up of the great plateaux has taken an enormous time, 
That the lava-streams were not poured out one upon another continuously 
is evident, among other things, from the weathered products found between 
the basalts.’’ 
