Conglomerates, and Marls of Devonshire. 
25 
cases, be very readily removed by washing. Indeed the Sandstones, 
when broken down into sand, give off a great amount of mere 
colouring matter, though they still retain the same hue. 
That the red colour is essentially due to peroxide of iron there 
is no doubt ; it is not easy, however, to account for its presence, for 
it must be remembered that it is by no means a local phenomenon 
since the red colom-s of the Trias have a wide geogi-aphical range. 
Sir Charles Lj'ell, when treating of this question, says To account 
for deposits of red mud and red sand, we have simply to suppose the 
disintegration of ordinary crystalline or metamorphic schists. Thus, 
in the eastern Grampians of Scotland, in the north of Forfarshire, 
for example, the mountains of gneiss, mica-schist, and clay slate are 
overspread with alluvium, derived from the disintegration of those 
rocks ; and the mass of detritus is stained by oxide of iron, of 
precisely the same colour as the Old Red Sandstone of the adjoining 
Lovv'lands. Now this allu\ium merely requires to be swept down to 
the sea, or into a lake, to fonn strata of red sandstone and red marl 
precisely like the mass of the ' Old Red ' or ' New Red ' systems of 
England."- In order that this may furnish a solution of the problem, 
we must suppose micaceous or hornblendic rocks to have been as 
widely distributed as the Triassic deposits, or that the red rocks 
mark the direction of an oceanic cui-rent, of the age of the Trias, by 
which the pigment was transported and distributed from some exten- 
sive area or areas of decomposition or disintegration. 
Nor does it appear that this would meet all the difficulties. That 
the prevalence of red colours has not been confined to merely one 
period of the Earth's history is manifested by the convenient and 
necessary terms "Old" and "New Red Sandstone"; nor are we 
without e\ddence of the recurrence of the same hue in much more 
modem times. Sir C. Lyell, speaking of the Upper Eocene strata 
of Auvergne, says " The most remarkable of the arenaceous groups 
is one of red sandstone and red marl which are identical in all 
their mineral characters with the secondary New Red sandstone and 
marl of England. In these secondary rocks the red gi'ound is some- 
times variegated with light gi'eenish spots, and the same may be seen 
in the tertiary fonnation of freshwater origin, at Coudes, on the 
Allier."f- Hence the problem becomes threefold: — 1st. The wide 
geographical distributions of the red colour. 2nd. Its suspension 
dming long intervals of time. 3rd. Its recurrence. I confess I am 
not satisfied with any hypothesis yet proposed on the subject. 
Confining ourselves, however, to the rocks immediately under 
notice, two questions present themselves : 
1st. When did the deposit receive its prevalent colour ? 
2nd. How were the exceptional colours produced ? 
* " Manual of i:iementary Geology," 5tlj cd., page 344. 
t " Manual of Elementary Geology," 5th ed., page 199. 
