deposit of clay, c, which extends further, and is finally replaced by nearly pure carbonate of lime, b, which grows thicker farther from shore.
Still the question recurs, where was the land from which the materials were drifted? The slaty mountains of Cumberland, the Isle of Man, Cavan, &c., were perhaps above the water; but could they alone yield the materials for the argillaceous sediments, 1000 feet thick, of Enniskillen, Derbyshire, and Craven, even if we suppose them to have been much diminished by the operation? The Lammermuir mountains, to the north, seem not to be of such composition as would yield the coarse quartzose sandstones; we must therefore appeal to the Grampians or Scandinavian ranges, or finally close all further discussion, by admitting that tracts of land which supplied part of the sediments, mixed with the limestones of the carboniferous period, have disappeared from the Northern and Western Oceans.
The coal formation, lying above these limestones, appears in many cases (Yorkshire, Lancashire, &c.) to have been accumulated, or according to the other hypothesis, submerged, in estuaries or lakes: if so, the local origin of the materials must be sought around those lakes, and in one or more directions from those estuaries. If, as seems probable, the coal fields of Yorkshire and Lancashire were once united, as those of Durham and Newcastle still are, the margins of the estuary in which they were formed are lost, except toward the mountains of Lancashire and Westmoreland. In like manner, no margin can be fixed for the estuary of the coal fields of Durham and Newcastle, except the Lammermuir range; and thus we are again conducted to the conclusion, that, unless those mountains be thought to have yielded all the sediments, great displacements of the crust of the globe have confused the ancient boundaries of the carboniferous sea, and reduced to mere conjecture the extent of the bordering land, and the circumstances of its drainage. This important,
