To illustrate my meaning:  let one place any hard substance, consisting of large fragments, in a mortar, and proceed to reduce it with a pestle to a fine powder.  The work proceeds rapidly at first, until a portion of the material is triturated; you then find that the pulverized part has packed around and protected the larger fragments, and the work is brought to a stand-still.  You have to remove the finer material if you would crush the pieces that remain.

The sea does not separate the sand from the gravel; it places all together at elevations where the waves can not reach them: 

“Waves or shallow soundings have some transporting power; and, as they always move toward the land, their action is landward.  They thus beat back, little by little, any detritus in the waters, preventing that loss to continents or islands which would take place if it were carried out to sea."[1]

The pebbles and gravel are soon driven by the waves up the shore, and beyond the reach of further wear;[2] and “the rivers carry only silt to the ocean."[3]

The brooks and rivers produce much more gravel than the sea-shore: 

“The detritus brought down by rivers is vastly greater in quantity than the stones, sand, or clay produced by the wear of the coasts."[4]

[1.  Dana’s “Text Book,” p. 288.

2.  Ibid., p. 291.

3.  Ibid., p. 302.

4.  Ibid., p. 290.]

{p. 80}

But it would be absurd to suppose that the beds of rivers could have furnished the immeasurable volumes of gravel found over a great part of the world in the drift-deposits.

And the drift-gravel is different from the gravel of the sea or rivers.

Geikie says, speaking of the “till”: 

“There is something very peculiar about the shape of the stones.  They are neither round and oval, like the pebbles in river-gravel, or the shingle of the sea-shore, nor are they sharply angular like newly-fallen débris at the base of a cliff, although they more closely resemble the latter than the former.  They are, indeed, angular in shape, but the sharp corners and edges have invariably been smoothed away. . . .  Their shape, as will be seen, is by no means their most striking peculiarity.  Each is smoothed, polished, and covered with striæ or scratches, some of which are delicate as the lines traced by an etching-needle, others deep and harsh as the scores made by the plow upon a rock.  And, what is worthy of note, most of the scratches, coarse and fine together, seem to run parallel to the longer diameter of the stones, which, however, are scratched in many other directions as well."[1]

Let me again summarize: 

I. Comets consist of a blazing nucleus and a mass of ponderable, separated matter, such as stones, gravel, clay-dust, and gas.

II.  The nucleus gives out great heat and masses of burning gas.