A. R. Horwood—Origin of the British Trias. 461 
occasional pebbles) in the centre, and finer and finer marls succeed in 
the last phase, which becomes increasingly ferruginous, as it merges 
into the lake-phase of the delta period. 
3. The oldest member of the series, the Bunter, is acknowledged to 
be a delta formation—as Professor Bonney showed many years ago— 
and there is no evidence for the discontinuity of the agency producing 
that mode of deposition. 
4, The continuity of the Bunter and Keuper is an argument for 
the extension of delta conditions to the Keuper, some ‘basement beds’ 
being indistinguishable from the Bunter. 
5. The general evidence of an oscillation of level in early Triassic 
times and of overlapping is a proof of aqueous agency. Coupled 
together these vertical and horizontal movements are more distinctive 
of fluviatile than lacustrine or marine conditions. 
6. There is a close analogy between the contour or geographical 
configuration of the Trias (whether we consider concealed or exposed 
areas) and modern deltas, e.g. the Mississippi, with its dactyloid 
extensions beyond the head. 
7. There is a distinct analogy between the regular alternations of 
pebbles or sand and marl and seasons of torrential rains and floods or 
drought; that is to say, one sort of sediment is brought down at one 
period of the year, another at another. This may be witnessed in 
modern accumulations such as those of the Nile or Mississippi, where 
floods occur. These alternations are due to overflow of banks where 
‘skerries’ le on the hilly grounds now, just as they did when they 
were deposited. The grey marl is heavier than the red, and deposits 
are arranged as in a diffusion column. 
8. The coloration of the Trias is original; that is to say, the red 
colour, imparted by peroxide of iron, was deposited on sediments 
under water-level. But it is not continuous everywhere with the 
bedding. ‘Catenary’ bedding is thus illusory. In only one case has 
an anticlinal fold of red-coloured marl been noticed underlying a grey 
band, but, on the other hand, synclinal folds are not uncommon, as in 
catenary bedding, the grey (heavier) marl lying in the hollows. 
9. The great thickness of the Bunter pebble beds is a proof of 
a subsiding area at the opening of the Trias and of conditions suitable 
to a gradually widening and deeper delta area. 
Normally delta deposits lie at an angle of about 45 degrees with 
the river bed, but as they are deposited in a subsiding area these beds 
describe an angle of 45 degrees and become horizontal. Thus the 
absence of delta bedding (not everywhere, for it occurs in Bunter, 
Lower and Upper Keuper here and there) in the Trias is proof that it 
was deposited in a subsiding area. 
The ‘radial dip’ around submerged areas is due to the ‘angle of 
rest’ which normally produces inclined beds. The winding of the 
course of a river like the Mississippi, producing wide alluvial plains, 
would account for the Red Marl being deposited much as in a 
lacustrine area. 
10. There is evidence from analogy of the ferruginous nature of 
the Upper Coal-measures, Permian, and Trias of the delta origin of 
the red marls. 
