OBSERVATIONS ON THE STRUCTURE OF STRATA. 
165 
the doubtful strata by their strike, to a less disturbed locality, a 
reference to lithological character and organic remains, or even to 
the original structure of the rock itself, should the ripple-mark or 
drifted ridges be well exhibited in the bedding. 
By the last of these methods the sequence of the strata is de¬ 
duced from the relation of their component laminse, either on the 
principle that the layers are attenuated from the top towards the bot¬ 
tom, or that newer groups rest unconformably upon older ones. And 
that such is very constantly the fact, may be shown by observing 
that in the ripple-mark formed on the bed of a river, the matter 
eroded from each of the furrows becomes stratified on the sheltered 
side of the next adjacent down-stream ridge, and that during the 
transfer the coarser particles remain hindmost and uppermost, slop¬ 
ing at a steep inclination, whilst the finer portions are carried for¬ 
ward to a lower position, and lie nearly level. Under this process 
the drifted matter deposited will evidently thin out along the sur¬ 
face of repose, from the summit to the base; and as its profile will 
form a curved layer differently inclined to the horizon at different 
points, it is presumable that, in the progress of accumulation, the 
lower or level parts of newer masses will generally rest upon the up¬ 
per or highly inclined edges of older groups, and thus give rise to 
frequent unconformabilities. 
In illustration of these views, a few sketches are subjoined from 
observation. 
Fig. 1 is a section from the valley of a river about two miles 
north of Carndonagh, in the county of Donegal. In the lower 
