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Dominica Dominica is a superb example of an elaborately dissected, composite volcanic island. It is 27 miles in length, north and south, and 12 miles in width--a grand mountain range of impressively bold forms rising from the sea to an altitude of 4747 feet. Receiving an abundant rainfall it is covered with luxuriant vegetation. The western coast was viewed from the passing steamer on my outward voyage; and on the return, opportunity was taken, while the steamer lay at anchor off Roseau, the chief port on the southwest coast, to make a short automobile trip into the interior. The dissection of the island as a whole is so far advanced that it was not possible to determine how many volcanoes compose its axial range. Certain rather wide -spaced valleys on the west coast, consuming more and more of the initial volcanic slopes as their depth increases inland, have reduced the intervalley sectors, which must have originally headed far up toward the volcanic centers, to short sectors of buttress-like form. These culminate in summits about halfway from the shore to the island axis, where peaks of decidedly greater height still rise. It is probable that the buttress summits are connected with the axial range by sagging intervalley ridges, the degraded representatives of the upper and inner half of the original full-length sectors. The western half of the island thus seems to include a marginal range with short-sector summits of a considerable height, each of which, rising at the apex of a sloping isosceles triangle, has its base along the shore. It may be noted that the deeply dissected volcanic island of Huaheine, in the Society group of the Pacific, possesses a similar marginal range of triangular buttresses, of larger or smaller size according to the spacing of the valleys, all around its oval circuit; but there each buttressed summit looks down over a lower central area, which probably represents the hilly remains of a caldera floor or of a central mass of less resistance than the overlying lavas of the buttress range. In contrast to this centrally excavated island, the central peaks of Dominica are still so high that they dominate the summits of the marginal range, at any rate on the west side of the island. The northern and southern ends of the island present instructive features. Morne au Diable, a recent addition at the northern end, is a well individualized volcanic cone, about three miles in diameter and 2917 feet high; it is maturely dissected by radial, consequent ravines and well cut back around its exposed side in model-like sea cliffs, up to 1000 feet in height, at the end of each radial spur. Unlike the low, spur-end cliffs on St. Lucia and certain other islands, yet to be described, which are small affairs compared to the embayed valleys between them, the high, spur-end cliffs of Morne au Diable have such dimensions as to suggest that they, like the more mature cliffs of St. Helena, represent a measure of abrasion closely comparable with the measure of erosion represented by the valleys. The valleys ought to have hanging mouths, but, as they seemed to mouth at sea level, it is suspected that a slight submergence--perhaps due to Postglacial ocean rise --has taken place since the cliffs and valleys gained essentially their present forms. Although this cone is of more advanced dissection than Saba and has perhaps been slightly submerged, as just suggested, no reefs rise around its shore. Their absence may be plausibly explained by the abundance of its outwashed detritus, for in spite of its inferred slight submergence its valley mouths did not seem to be embayed. Furthermore, as it stands in the marginal belt of the Atlantic coral seas, a more potent cause of the absence of reefs is probably to be found in the insufficient rise of ocean temperature in Postglacial time. THE SOUTHWESTERN CLIFFS OF DOMINICA The southern end of the west coast of Dominica, sketched from the steamer at anchor off Roseau a little farther north, exhibits two great radial spurs--shown in the right half of the sketch--cut off in slanting terminal facets, thus repeating the features of Morne au Diable but on a larger scale as well as in a more advanced stage of sculpture. The more advanced sculpture is shown by the undulation of the spur crests and by the excavation of steep-pitching ravines, widening upward into spatulate forms, on the spur sides and on their faceted ends. A rather strong submergence is clearly indicated here by the separation of the spurs at the shore by valleys filled with sloping mud flows to a width of a quarter or half mile. The submergence thus inferred may well have been as much as 500 or 800 feet, and it must therefore be explained by island subsidence. If the island were restored to the higher stand it held during the production of the inter-spur valleys and spur-end cliffs, the measure of abrasion seen in the cliffs would, as in the case of Morne au Diable, appear to be comparable with the measure of erosion seen in the valleys; but in view of the mature stage of sculpture here attained before submergence, as compared with the immature dissection of Morne au Diable, it is probable that the sculpture of the southern end of the island was well advanced even before the recent cone at the northern end was formed. At that early time in the history of the island, the temperature of the surrounding sea must have been favorable to reef growth, in spite of the absence of reefs as indicated by the great cliffs of abrasion; for an early stage of this still lofty island must have been contemporaneous with a more advanced and reefdefended stage of a worn-down island like St. Lucia, to be described below. Hence the early absence of reefs around Dominica--or at least around its southern end--may be best explained by the abundance of detritus outwashed from its valleys. It is profitable to recall here that Tahiti, the chief island of the Society group, also exhibits spur-end cliffs of a size commensurate with its inter-spur valleys and that, in spite of its situation in the coral seas of the Pacific, its cliffs were cut at a time when neighboring older islands, like Borabora, were undoubtedly reefencircled; hence the absence of reefs around Tahiti during the abrasion of its great cliffs is to be explained by the outwash of detritus to its shores. Thus confirmation is found for the same explanation of the cliffs of southwestern Dominica. Furthermore, like Dominica, Tahiti has subsided since its cliffs were cut; but unlike Dominica, Tahiti is now surrounded by an up-built barrier reef. The reason for this contrast may be found partly in the mud flows by which the Dominica valleys have been deluged; for such flows are destructive of any preëxistent reefs, as will be seen on St. Lucia; but partly also in the situation of Dominica in the Atlantic marginal belt, where any Preglacial reefs that may have been formed would have been cut away by low-level abrasion in the Glacial epochs and where, even in Postglacial time, the temperature for reef growth has been reached tardily if at all. WEST COAST OF DOMINICA Several valleys of the west coast have benched lateral slopes, apparently the result of alternating epochs of mud-flow eruption and torrent erosion after the coast had been deeply dissected in the production of the buttressed marginal range above described. The benches would appear to represent successive mud flows, each of which occupies a valley cut in an earlier flow and which is in turn cut by a valley occupied by a later flow; except that the first and largest valley would seem to have been cut in the original slopes of the west coast. It would, perhaps, be safer not to imply by the word "successive" that all the flows subsequent to the deep dissection of the coast are now represented by valley-side benches. Many minor flows may have been buried under later and greater flows; and similarly, many little valleys in minor flows may have been entirely consumed in the excavation of greater valleys in greater flows. Hence the above description may be conservatively modified to read: The benches would appear to represent a series of decreasing maxima in an irregular succession of greater and smaller mud flows; and a similar phrasing would apply to the valleys. The most pronounced examples of these forms were seen in the large valley next north of the great faceted spurs. Other examples of fairly good definition were noted farther north; Roseau occupies the higher part of a torrent delta at the mouth of the benched valley next north of the one sketched. Fine sections of the mud flows were seen in the valley-side cliffs inland from Roseau. Along the west coast the shore of the inter-valley slopes, as well as that of the benches that slant forward from certain valleys, is generally cliffed to heights of 50 or 100 feet. This small measure of abrasion is by no means comparable with the work of near-by valley erosion. It may therefore be suggested that abrasion of the inter-valley shore hereabouts has been weakened or delayed by the issue of the bench-making mud flows from inland centers of eruption, as well as by the torrential outwash of detritus from the valleys; also that such cliffs as were earlier cut from time to time are now submerged by the subsidence of the island, evidence for which is found in the width of the mud flows on the southwest shore between the great faceted spurs, as told above. It may be added that the torrential outwash of detritus is still manifestly effective today in delaying abrasion, as is shown by the slightly convex advance of the Roseau delta farther seaward than the shore to the north and south. According to the chart, the east coast is more embayed and more cliffed than the west coast. Next: Dominica Bank Home