Conglomerates, and Marls of Devonshire. 
47 
same, the time-value of a rock of detrital origin is a direct function 
of the fineness of the grains composing it. 
Deposition cannot, on the whole, go on more rapidly than the 
denudation and trituration which necessarily precede it ; and 
when, with this fact prominently in view, we attempt to reduce to 
centuries the cubic miles of the Devonshire Trias, it is impossible 
to escape from the conclusion, that this fragment of one division of 
a single and comparatively unimportant unit in the chronological 
scale of the geologist, is worth vastly more than the few milleniums 
within which our fathers believed the age of our planet to be 
comprised. 
It is of no avail, nor is it a reply, to say that we are not possessed 
of a modulus by which to compare astronomical with geological 
revolutions. It is not at present, it perhaps never will be, in our 
power to say how many rotations on her axis, or how many revo- 
lutions round the sun were completed by our world, or, to take a 
much larger unit, how many times the pole of the equator passed 
through its circuit round the pole of the ecliptic, during the depo- 
sition of a bed of sandstone. Nor is it in our power to resist the 
conviction that the antiquity of the earth is vast beyond human 
comprehension. 
Nor will it more avail to invoke the aid of Convulsion — that 
forlorn hope of an age now hastening to extinction. The rocks 
under our notice are not only without indications of paroxysm, 
but they contain evidence that during their era events succeeded 
one another as tranquilly as at present. The rain-drop impres- 
sions in the marly sandstones tell us that then, as now, there were 
gentle showers, and the desiccation cracks indicate the sweet 
shining of the sun after the shower ; the ripple-marks — obviously 
not such as can be ascribed to ocean currents in deep water — 
show us how gentle were the Triassic wavelets, and how carefully 
the returning tide preserved the inscriptions of its immediate 
predecessor. These indications of a tidal strand are capable of 
giving us still further information. They are found in every part 
