A, EF. Verrill—The Bermuda Islands; Geology. 63 
When the percolating waters meet a nearly horizontal layer of 
impervious red clay, or a very compact layer of fine shell-marl, its 
downward course being’ arrested, it may collect in and consolidate 
more firmly the layers just above. The layers of hardened limestone 
will also vary in hardness according to the fineness and compactness 
of the shell-sand and calcareous mud composing them ; according to 
their inclination and drainage ; according to the amount of percolat- 
ing water ; and also according to their depth beneath the surface. 
Some of the beds of sand, even of considerable thickness, are still 
Figure 9.—Cathedral Rocks on Somerset Island; the ruins of an ancient cavern 
and water passages, partly broken down and dissected by the sea. The roof 
has partly fallen. The coiumns are hardened by infiltration and roughly 
pitted. The bottom, which is above high tide, is covered with shell-sand. 
loose, with little or no consolidation, although of ancient origin with 
thick deposits of hard limestone rocks over them. Sometimes irreg- 
ular masses or “ pockets” of the loose sand occur in the harder lime- 
stones, fig. 4. When such loose deposits of sand happen to become 
exposed in the shore cliffs the soft contents are quickly washed away, 
leaving grottoes or cavernous places, large and small, in the cliffs. 
Probably the remarkable “Cathedral rocks” (fig. 9), on the west 
shore of Somerset Island,* have been formed mainly by the rapid 
* See pl. xxi; also these Trans., pp. 427, 473, and pls. Ixxxviii, lxxxix ; ‘‘ The 
Bermuda Islands,” pp. 15, 61, the same plates, 
