Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/526

 to the west, that the descent in the former direction is abrupt, in the latter gradual, and that from Senafe, where the force first reached the highlands, lat. 14° 40' N., to south of Antalo (about 13° 10') the country is comparatively entirely open, with but few ascents or descents of importance, of an average height of nearly 8000 feet above the sea, and composed principally of metamorphic rocks, limestone, and sandstone ; whilst from a few miles south of Antalo the road traverses a succession of passes from 9000 to 10,500 feet above the sea, and the only rocks seen are volcanic, belonging to two distinct groups of bedded traps.

Metamorphic rocks. — These form the base of the greater portion of the Abyssinian plateau, if not of the whole. As usual, they vary to a very great extent in mineral character and degree of metamorphism, some being highly crystalline and even porphyritic granites, others mere slaty schists still exhibiting the original bedding. No reason was observed for dividing them into several groups ; but it is possible and even probable that more extensive study would justify such division.

The direction of the foliation is very constant. From Af Abed in Habab (lat. 16° 10' N.) to about 25 miles south of Adigrat in Tigre (about 13° 50'), upwards of 150 miles, it rarely diverged more than a few degrees from due north and south. The dip is not quite so constant ; but it is usually nearly vertical, though exceptions, probably due to disturbance, occur, at the base of the outer ranges especially. In the Anseba valley and around Keren in Bogos the strike of the foliation changes to north-east and south-west.

This strike is clearly due to cleavage, and not to original bedding ; for in some very slaty rocks near Senafe the original bedding was observed crossing the foliation. The same might also have been inferred from the regularity of strike and high dip.

This strike corresponds with many important features of the country, such as the north and south ravines which cut through the outer ranges and tend to so great an extent to facilitate access to the highlands.

Adigrat Sandstone. — This is a massive sandstone, generally white in colour, of considerable thickness, which caps many of the higher ranges in Agame, and the steep white scarps of which are perhaps the most striking natural feature in northern Tigre. It is in places about 1000 feet in thickness, but usually less. No fossils have been found in it, and its age is uncertain. Messrs. Ferret and Galinier class it, though with doubt, as Tertiary ; but as I found it to all appearance distinctly passing beneath the oolitic limestone of Dongolo south of Adigrat, it is evidently of older date. It is far from clear whether the limestone rests conformably upon it ; probably it does not do so, as otherwise it is difficult to understand the total absence of the limestone about Adigrat and Senafe, where trap alone rests on the sandstone. It is possible that the coal-bearing sandstones known to exist west of Lake Dembea may belong to the same series as the Adigrat sandstone.

The Adigrat sandstone always rests on metamorphic rocks. It is