We return again to the rocky ledge in mid-channel, and proceed to ascend the course of the stream, marking as we go the changes in the character and features of the stone that forms the cliff on either hand. We come to a bare part of the ravine where brushwood and herbage find but a scanty footing and where accordingly the rocks can be attentively studied. The face of the escarpment shows a number of beds of pale grey sandstone alternating with courses of a dark crumbly shale. The sandstones being harder and firmer in texture stand out in prominent relief while the shales between have been wasted away, covering the bottom of the slope with loose debris. We can mark too that, as this decay goes on, the harder beds continually lose their support, cracking across chiefly along the lines of joint, and rolling down in huge angular blocks into the stream. In truth we cannot doubt that every year adds to this decay and thus slowly widens the dell, for the broken fragments do not form in heaps over the solid rock below so as to protect it from the weather, but are evidently carried away by the stream and hurried down the ravine onwards to the sea. From what has been said above relative to the disintegration of rocks by percolating water, frosts, and other causes, the reader will easily see how this rotting away of the sides of the ravine must be carried on; and he will not fail to mark that here we have at work an agency not yet considered, that of running water. The effects of the weather are seen in the crumbling, ruinous cliffs overhead; the effects of the streamlet are observable in the continual removal of the rubbish whereby a fresh surface is ever exposed to the decomposing forces, while at some points we can mark the water actually undermining an overhanging part of the cliff from which there are ever and anon vast masses precipitated into the channel where eventually they get worn down and carried away out to sea. "Still," you may remark, "these forces are at work only in widening a channel already made. How was the ravine formed at first?"