Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/527

 succeeded by limestone, as already mentioned at Dongolo ; but near Adigrat it underlies trap, probably of the Magdala group, with apparent conformity.

Antalo limestone. — The next group in ascending order consists chiefly of limestone ; and in this, for the first time, fossils occur in some abundance. The rock is pale-coloured and usually well stratified ; it occupies the line of route taken by the British force for about 70 miles from Dongolo till south of Antalo. To the north scarcely any other beds are interstratified ; but further south, especially near Antalo, both sandstones and traps are met with, the latter being always doleritic, and a portion of the bands being, to all appearance, contemporaneous. The uppermost part of the formation is of sandstone and conglomerate, which are best seen in the Mishek valley south of Antalo.

These limestones have been very well described by Messrs. Ferret and Galinier, who quite rightly attributed them to the Jurassic formation. Although fossils abound, it is singularly rare to find any in sufficiently good preservation for accurate determination ; but amongst those obtained are the following : —

Hemicidaris, sp, nov., near H. Luciensis, D'Orb., and H. Wrightii, Cotteau.

Trigonia costata, var. pulla, Sow.

Modiola Baini, Sharpe, or a closely allied species.

Ceromya concentrica, Sow.

C. similis, Sow.

C, sp., perhaps a var. of C. excentrica, Sow.

Pholadomya sp., near Ph. recurva, Ag., and P. concatenata, Ag.

Ph., sp. near Ph. (Panopcea) punctifera, Buv.

Alaria, sp.

Cerithium, sp.

and numerous casts of bivalves.

Other forms occur ; but the above are the most characteristic.

Trappean series. — Nearly all southern Abyssinia, with Shoa, is known to consist of bedded traps ; and although they do not occupy so extensive a surface in Tigre, they nevertheless cover a very considerable tract.

On the road to Magdala volcanic rocks were met with at Senafe, where several hills consist of basalt and trachyte. The latter passes into a claystone which simulates sedimentary rocks to a remarkable extent, and forms the singular bosses known as Senafe rocks, which were mistaken for sandstone by almost all observers ; indeed I was myself deceived at first. Near Adigrat is a great range formed of horizontal beds of basalt, and probably, in the upper portion, which I had no opportunity of visiting, of trachyte ; but it was south of Antalo that the great trappean area was entered, and traversed by the route of the army, without a break, to Magdala.

This trappean series plainly consists of two groups, the upper of which was clearly seen resting unconformably on the lower in the valleys of Atala and Ayba south of Antalo. The lower division I propose to call the Ashangi group, from its great development around the little lake of that name ; the higher the Magdala group.