Basalts Part 2 – Tertiary Volcanic Terrains of Tigrinyaland and Semien

In the vicinity of Aksum, the basalt often presents large crystals of labradorite. Its compact variety gives rise to a basaltic phonolite.

In Shiré, the basalt forms different cones, notably in the vicinity of Selekhlekha, Adi-Bellesss, in Tsimbilla and at Adi-Bahra, etc. At Selekhlekha, we see a brown scoriaceous basalt with amygdals of bluish chalcedony quartz agate, surrounded by a greenish material (altered pyroxene), generally hollow and are often covered with acicular radiated mesotype. This basalt also contains some small vitreous crystals independent of the essential composition of the rock. In the vicinity of Adi-Bellesss, the basalt is black grainy, or else it is a brown scoriaceous basalt, marbled by a green material and by quartz agate chalcedony. It also contains white and yellowish zeolites. At other times, we see a green rock, more or less spilitic and which is composed of chlorite baldogée (green earth) and chalcedony agate quartz. These basalts therefore contain more or less considerable amygdals of chalcedony or resinite quartz which are sometimes reddish brown, sometimes milky, sometimes prases.

The chalcedony in geode is often covered with crystals of hyaline quartz. These minerals are so abundantly distributed in the basalt that the sedimentary Tertiary terrains of the vicinity of Aksum are covered with them. It is probably the alluviums that detach them from the basalt and which bring them to this locality.

At Firafira, the basalt forms a cone above the Tertiary clays which it has converted into porcellanites.

Near Cheleqot, the basalt has separated the layers of the oolitic terrain to occupy the bottom of the Mai-Cheleqot valley. It has also pierced on the right and left the same terrain which it has partly covered. This basalt has modified the rocks of the oolitic terrain as well as those of the Tertiary terrains, because it has made them more or less jasper-like.

In the Semien¹, the basalts emerged in the middle of the Tertiary sedimentary terrains, and probably they also cross the trachytes which are found at their base. In the Semien, as in Tigray² and Shiré, we see, at Maitalo, an amygdalar basalt with zeolites, or a blackish green basalt. Near the Lemalimo, we observe an amygdalar basalt with concentric brick-red agates, or else a black basalt with zeolite crystals, or else a retinite dependent on the basalts.

At Mount Buahit, a scoriaceous basalt with zeolites is seen. At Mount Dejen, we observe a very peridotose brownish scoriaceous basalt. At Noari, an amygdalar basalt with crystals of mesotype is seen. In these basalts, or at least on their limits with the layers of sandstone and clay, we also find quartz resinite of a beautiful green. Finally we see near Maitalo a very soft green substance in amygdals in the basalt. This substance, which one could take at first glance for turquoise, is formed in large part by chlorite baldogée (green earth), which results from the alteration of pyroxene.

It would be of the greatest interest for the geologist to be able to establish directional relations between the various deposits of trachytes and basalts of Abyssinia³, as well as with those of the coasts, either of the Indian Ocean or of the Red Sea. But we lack sufficient data to arrive at a serious result. So we have abstained from any reflection on this subject.

This is the eighteenth installment of the geological description of Tigrinyaland⁴ and Semien, which has been adapted from Messrs. Ferret and Galinier‘s work published in 1847.⁵

Reference Notes

1. Semien was historically the frontier province of the Tigrinya with the Amhara. However, since the reign of Emperor Susenyos, the province of Semien had been governed by members of the Amhara royalty and nobility. Following the death of Dejazmatch Sabagadis in 1831, Semien under its Amhara ruler Dejazmatch Wubbe turned from the frontier province of the Tigrinyaland to the power-center of the Tigrinyaland. This continued until the rise of Emperor Tewodros in 1855. ↩︎
2. The Tigray Province was bounded on the east by the province of Agame, on the west by the province of Shire, on the south by the River Wari, and on the north by the River Mereb. ↩︎
3. Abyssinia refers to the region which encompasses the modern-day states of Ethiopia, Eritrea, Djibouti, parts of Sudan and parts of Somalia. ↩︎
4. Tigrinyaland was a collective name of the Midri-Bahri (modern-day state of Eritrea) and Tigray (the northernmost region of modern-day Ethiopia). The term employed for Tigrinyaland by Messrs. Ferret and Galinier in their book is “Tigré”, which had been the designation used by the Amhara rulers of Abyssinia to refer both to the Tigrinya people and the Tigrinyaland. ↩︎
5. Ferret, Pierre Victor Adolphe et Galinier, Joseph Germain (1847) Description Géologique du Tigré et du Samen. Voyage en Abyssinie dans les provinces du tigre, du samen et de L’amhara. Tome troisième. Paulin: Paris. ↩︎