tAMPLUGH : NOTES ON THE WHITE CHALK OF YORKSHIRE. 181 
Great Tlioriiwick at low water, have already been outlined. It illus- 
trates the formation of caves and fissures by the action of the waves 
along vertical joints. As I showed in last year's number of our Pro- 
ceedings, these joints are frequently small faults of a few inches' throw. 
In such cases there is usually a slight crushing of the adjacent 
surfaces, rendering the rock less resistant. Besides the direct scour- 
ing action of the waves, it is very certain that they exert a consider- 
able indirect effect in alternately forcing before them air under 
pressure into the fissures, and then withdrawing it in their rapid fall. 
In many spots along these tunnelled cliffs one may see in stormy 
weather the forceful puffing out of steam-like bodies of spray from 
crevices communicating with the caverns into which the great waves 
are rolling. 
It is no doubt in this way quite as much as by the action of frost 
that the domes of the caves are sometimes heightened far beyond 
the direct wave erosion. 
7. Western Side of North Sea Landing. 
This and the following pictures of the series serve to illustrate 
the details of the jagged coast between North Sea Landing and the 
extremity of the headland, and are especially instructive as to the 
working methods of the agencies of erosion. 
This view shows the natural arches, named on the old 6-inch 
map ' Bacon Flitch Hole,' on the western side of North Sea Landing, 
the standpoint being on the top of a spur of the adjacent chff, 
looking northward. The height of the cliff is about 70 feet. It con- 
sists of Chalk-with-Flints, with a capping of Glacial drift. Fossils 
are rare in this portion of the Flinty Chalk, and very difficult to 
extract on account of the hardness of the rock. There occurs, however, 
in the corner of the Landing a little to the southward of this section, 
a thin seam containing fragments of the stems or arms of a crinoid 
(probably Bourgueticriniis) in abundance. The main archway has 
evidently been excavated along the joint ("or joint-fault) plane which 
is seen above the crown of the arch on the right-hand side. One can 
see that in the course of time the crumbling down of the rock along 
this plane will bring about the destruction of the arch and the sever- 
ance of the outer mass from the inner cliff. An outstanding stack 
like those of the next three plates will thus be formed. 
