The depression between the escarpment and the drift-mounds is a remarkable feature, and suggests an entire change of conditions since the boulder-beds were deposited. If they are a mere sub-aërial talus, they should rest directly against the cliff face, and the depression separating them must have been formed after the accumulation had ceased; and yet no satisfactory reason can be assigned for such cessation, if rain and river action were the only operating causes. The form of the mounds in the valley west of Tasseremout at once conveyed to me the impression that they were of glacial origin; and the discovery of undoubted moraines in the higher valleys strengthened my conviction that the boulder-mounds and ridges flanking the Atlas plateau can only be satisfactorily explained as the result of glaciers covering the escarpment, leaving on their recession the intermediate depression.
(b) Moraines of the Higher Atlas.—Kindred phenomena occur higher up in the Atlas valleys, most notable in the case of unquestionable moraines, commencing at the village of Adjersiman, in the province of Reraya, at an altitude of 6,000 feet. Here we met with a gigantic ridge of porphyry blocks, having a terminal angle of repose of between 800 and 900 feet in vertical height, and grouped with several other mounds and ridges of similar scale, all composed of great masses of rock with little or no admixture of small fragments, and completely damming up the steep ravine and retaining behind it a small alluvial plain 6,700 feet above the sea-level.
We failed to detect any scratched blocks or striæ; but that these ridges are true glacial moraines no one who has seen them and compared them with other glacial phenomena would for a moment doubt; and their interrupted occurrence at various heights is strictly in accordance with the distribution of moraines in many of the Swiss and Scotch valleys.
Lieut. Washington, in referring to the pointed mountainous hills NW. of the city of Marocco, crossed on his homeward journey, describes one of them as being ‘covered with masses of gneiss and coarse-grained granite (? diorite), many of the blocks being several tons in weight,’ and asks, ‘how got they there?’ ‘If granite, the nearest granite mountains are at a distance of twenty-five to thirty miles: can they be boulders?’ As far as my own observations go, there was no rock in situ in the part of this range I visited near Marocco resembling granite or diorite; and in connection with the boulder-mounds of the Atlas, the occurrence of foreign blocks north of the plain of Marocco so far from the parent source, is a circumstance of great interest.
(c) Stratified Red Sandstone and Limestone Series.—A long line of comparatively low and flattish hills, forming a plateau, with an average height of about 4,500 feet above the sea, and 2,800 feet above the plain of Marocco, intervenes between it and the main ridge of the Atlas. The edge of this plateau facing the plain is for some distance an escarpment, exposing stratified beds of limestone containing bands of chalcedonic concretions, underlain by grey and puce-coloured marls. As this plateau is crossed from north to south towards the Atlas ridge, its central line would represent a synclinal, from which the beds rise northwards towards the plain and southwards towards the Atlas; but it is locally broken and contorted, and near Tasseremout the limestone beds stand up nearly on end. South of the synclinal, i.e. between the centre of the irregular plateau and the Atlas, great deposits of red sandstone and dark-red conglomerate, interstratified with cream-coloured shelly limestone, occur, which appear to be inferior members of the series of limestones and marls exposed in the escarpment facing the plain. Lieut. Washington, who ascended Miltsin to a height of 6,400 feet, describes hard red sandstone with an east and west strike dipping 10° south, as occurring at this elevation, which is nearly 2,000 feet higher than we observed the Red Sandstone series in the province of Reraya farther west, and also both in his approach and descent from Miltsin of ranges of limestone running NE. and SW. dipping 70° SE. with abrupt sterile sandstone mountains rising above them. From the few obscure fossils, including an Ostrea, I was able to collect from the limestone bands, Mr. Etheridge considers that they are of Cretaceous age. They are, like the beds of the plain, remarkable for containing great deposits of chalcedonic concretions; but the latter may possibly be of more recent age. They rest unconformably on the upturned edges of grey shaly beds, and extend also over the porphyries that form the great mass of the Atlas chain. They appear to have been deposited subsequently to the porphyry ridge assuming its present hill-and-valley contour, as little isolated fragments are seen clinging to the sides of a narrow ravine leading out of the valley we ascended through the province of Reraya to the Atlas. Their relation to the few exposures of stratified beds in the plain is somewhat uncertain, as no fossils were obtained in the latter, and there are no direct connecting links; but, judging from petrological similarity, and from the fact that Neocomian fossils occur in exposed beds on the coast cliffs, and Cretaceous fossils in the beds forming the crest of the plateau, it seems possible that an unbroken series occurs from the cliff north of Saffi to the plateau skirting the Atlas, representing the whole of the Cretaceous epoch; but it is also open to question whether the level beds of the plain may not be an inland extension of the strata of Miocene age from which Dr. Hooker obtained fossils at the Jew’s Cliff south of Saffi.
(d) Grey Shales.—At several points on entering the lateral valleys of the Atlas, almost vertical shaly beds are crossed, having a strike nearly east and west, corresponding with the trend of the chain. They clearly underlie, and are unconformable to, the Red Sandstone and Limestone series; and their almost vertical position appears connected with one of the several upheavals that have affected the chain. Of their geological age there is no evidence, except that they are pre-Cretaceous. In places, as at Assghin, they abound in nodules of carbonate of iron. Pale shales, containing quartz veins, crop up near the village of Frouga, in the plain south-west of Marocco, which may possibly belong to this series; and if the porphyries forming the mass of the Atlas are contemporaneous, they are probably interbedded with these grey shaly beds. Lieut. Washington speaks of the occurrence of clay-slate dipping 45° east between El Mansoria and Fidallah, and again of a hilly country of clay-slate near the plain of Smira, and at Peira, farther south; but it is impossible to say whether these beds are related to the grey shales of the Atlas.
(e) Metamorphic Rocks.—The most important development of metamorphic rocks in the neighbourhood of Marocco is on the north side of the city. In its immediate neighbourhood, three miles to the north-west, a low rugged hill occurs, composed of a very hard and compact dark-grey rock, containing knotted white concretions elongated in the line of stratification, which dips from 50° to 80° south-west, the strike being north-west and south-east. The whole of the north side of the plain is bounded by ranges of rugged hills of similar form, and apparently rising from 2,000 to 3,000 feet above the plain. We had not an opportunity of visiting them; but, judging from their outline, they are identical in formation with the hill close to Marocco. We observed nothing in the Atlas resembling it. Lieut. Washington, who crossed these hills on his journey to Marocco at about the point I visited, and again forty miles to the east, near the source of the river Tensift, on his homeward journey, speaks of them as from 500 to 1,200 feet in height, consisting of micaceous schist and a schistose rock with veins of quartz dipping 75°, with a strike north by east and south by west. The strike may vary a little at different points, and, taking Lieut. Washington’s and my own observations together, would average about north and south; and it is worthy of note that these apparently ancient rocks are nearly at right angles to the strike of the rocks of the Atlas chain a few miles to the south.
The only other metamorphic rocks that came under our notice were:—first, white marble or metamorphic limestone, intercalated with the porphyrites at the summit of the ridge of the Atlas south of Arround; secondly, mica-schists, pierced by red porphyry dykes, forming the mass of Djebel Tezah, a peak 11,000 feet in height, and fifteen miles farther west, ascended by Dr. Hooker and Mr. Ball after my return. It is possible that the mica-schists may be a portion of the grey-shale series, metamorphosed by the intrusion of the porphyry dykes. Lieut. Washington, on his first day’s journey south of Tangier, refers to the occurrence of rounded schistose hills about 300 feet high, strike north-west and south-east, dip 75° south-west, containing mica-slate with veins of foliated quartz; but I have no recollection of observing any such metamorphic rocks between Tangier and Tetuan.
(f) Porphyrites.—Of the eruptive rocks of the Atlas, porphyrites and porphyritic tuffs occupy by far the most prominent position, forming the great mass of its ridge.
On entering the lateral valleys, after crossing the vertical shaly beds, great masses of red porphyrites and tuffs are met with, associated with specular iron and occasional green porphyries. The harder portions of the latter are seen as Verde antique pebbles in the river-beds; but we failed to detect this in situ. From the large proportion of tuffs that occurs the porphyrites appear to be interbedded, and are possibly contemporaneous with the vertical grey shales to which they are adjacent. They are overlapped unconformably by the Red Sandstone and Limestone series of Cretaceous age. The late Mr. D. Forbes informed me that they bear a strong likeness to the porphyrites of the Andes, of Oolitic age; but beyond the fact that they were in existence and had undergone denudation into hill-and-valley contour before the Cretaceous beds were deposited over them, there is no certain evidence as to their age.