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miles long, from 5 to 12 miles wide, and from 5000 to 6000 feet deep." From our present point of view the chief lesson which we derive from a study of the Grand Canon district is simply this: that horizontally arranged strata tend under the action of epigene agents to form flat-topped mesas and pyramidal hills and mountains. The contours of those prominent features and the detailed sculpturing of cliffs and rock-terraces will depend largely upon the character of the strata out of which the hills and mountains are carved, and also to a great extent upon the climate. In a dry elevated tract like that of the Canon district the influence exerted by the petrological character of the strata in determining the detailed features of the ground is everywhere conspicuous. In other regions where moister climatic conditions prevail this influence, although never absent, is yet not so strongly marked.
In the foregoing discussion the configuration assumed by horizontal strata has been dealt with in such detail that it is not necessary to cite more than a few other examples to show that wherever the same geological structure occurs denudation has resulted in the production of similar land-forms.
The lonely group of the Faroe Islands, lying about half-way between Scotland and Iceland, are the relics of what at one time must have been a considerable plateau. They extend over an area about seventy miles in length from north to south, and nearly fifty miles in width from east to west. The original plateau could not have been less than 3500 square miles in extent. But as the islands have everywhere experienced excessive marine erosion, it is certain that the plateau out of which they have been carved formerly occupied a much wider area. The geological structure of the islands is very simple. They are built up of a great succession of basalts with thin intervening layers of tuff (volcanic dust, etc.) arranged in approximately horizontal strata. The islands are for the most part high and steep, many of them being mere mountain-ridges that sink abruptly on one or both sides into the sea. The larger ones show more diversity of surface, but possess very little level land. All have a mountainous character, and, owing to the similarity of the rocks and their arrangement, exhibit little variety of feature. They form as a rule straggling, irregular, flat-topped masses, and sharper ridges, that are notched or broken here and there into a series of isolated peaks and truncated pyramids. Sometimes the mountains rise in gentle acclivities, but more generally they show steep and abrupt slopes, which in several instances have inclinations of 250 to 270 or even 300. In many places they are yet steeper, their upper portions especially becoming quite precipitous. They everywhere exhibit a well-marked terraced character; precipices and long walls of bare rock rise one above another, like the tiers of some cyclopean masonry, and are separated usually by short intervening slopes, sparsely clothed with grass and moss, or sprinkled with tumbled rock-rubbish. The coasts are usually precipitous, many of the islands having only a few places where a landing can be effected. Not a few are girt by cliffs, ranging in height from 200 or 300 feet up to 1000 feet, and even in some places exceeding 2000 feet. The best-defined valleys are broad in proportion to their length. Followed up from the head of a sea-loch, they rise sometimes with a gentle slope until in the distance of two or three miles they terminate in a broad amphitheatrelike cirque. In many cases, however, they ascend to the water-parting in successive broad steps or terraces. Each terrace is cirque-shaped, and framed in by a wall of rock, the upper surface of which stretches back to form the next cirque-like terrace, and so on in succession until the series abruptly terminates at the base, it may be, of some precipitous mountain. Occasionally the neck between two valleys running in opposite directions is so low and flat that it is with difficulty that the actual water-parting can be fixed. In such cases we have a well-defined hollow, bounded by precipitous, terraced hill-slopes, crossing an island from shore to shore. Were the land to be slightly depressed such hollows would form sounds separating adjacent islands, while the valleys that head in cirques would form sea-lochs. There can be no doubt, indeed, that the existing fiords of the Faroes simply occupy the lower reaches of land-valleys, and that the sounds separating the various islands from each other in like manner indicate the sites of long hollows of the character just described. In a word, the islands are the relics of a plateau of comparatively recent geological age, for the rocks date no further back than Oligocene times. All the land-features are the result of subaerial erosion guided and determined by the petrological character and horizontal arrangement of the strata. The precipitous cliffs of the coast-line owe their origin, of course, to the undermining action of the sea, the rocks ever and anon giving way along the well-marked vertical joint-planes.
In Great Britain horizontal strata occupy no broad areas. But wherever they put in an appearance they yield the same surface-features. Thus in the north-west Highlands we have the striking pyramidal mountains of Canisp, Suilven, and Coulmore, carved out of horizontal red sandstones of Pre-Cambrian age. In Caithness, again, we have the peaked and truncated pyramids of Morven, Maiden Pap, and Smean, hewn out of approximately horizontal Old Red Sandstone strata. Ingleborough is another good example of a pyramidal mountain having a similar geological structure. Many illustrations are likewise furnished by the horizontal strata of other lands. Thus pyramidal and more or less abrupt hills, the precipitous sides of which are defined by vertical joints, are common in the horizontally bedded " Quadersandstein" of Saxon Switzerland. So again in the region of the Dolomites, whenever the strata are horizontal the mountains carved out of them tend to assume pyramidal forms. In a word, we may say that all the world over the same geological structure gives rise to the same land-forms.
River-courses hewn in horizontal strata will vary somewhat in form according to the nature of the rocks and the character of the climate. In regions built up of relatively unyielding rocks, or of alternations of these and less resisting beds, the valleys tend to be trench-like, and the mountain-slopes are more or less abrupt. But under the influence of rain, springs, and frost these harsh features are toned down, river-cliffs are benched back, and abrupt acclivities are replaced by gentler slopes. Should the strata consist of soft materials throughout, there will be a general absence of harsh features; round-topped hills and moderate valley-slopes will characterise the land. Nevertheless, whether the strata be "hard " or "soft," thick-bedded or thin-bedded, or show alternations of many different kinds, and whether the climate be arid or humid, equable or the reverse—tropical, temperate, or arctic—the same general type of surfacefeatures can always be recognised.