Basalts Part 1 – Tertiary Volcanic Terrains of Tigrinyaland and Semien

Basalts are more or less crystalline rocks composed essentially of pyroxene, augite, labradorite, and peridot. Accordingly, we shall associate with the basalts many rocks which, at first sight, seem to be different, but which are actually accidents of basalt.

Basalts are much more developed in Abyssinia¹ than the trachytes. Consequently, their emergence has had a greater influence on the relief of the soil, and has contributed more powerfully to the modifications which the other rocks, principally the sandstones and the tertiary clays, have undergone in their vicinity. The basalts form the highest points of Abyssinia, and are frequently covered with snow.

In Serayé, from Adi-Ghebray to Gundet, the basalts constitute the surface soil, either of the plateau with its hillocks, or of the massif on which Gundet is built. In the latter place, they cross the syenite, as often happens in Shiré.

At Adi-Bahro, we see sometimes a greenish-black grainy basalt, sometimes an amygdaloid wacke basalt, containing zeolites and chalcedony agates, sometimes a reddish wacke basalt, amygdaloid and zeolitic.

At Mai-Libus we observe a fine-grained peridot rock (peridotite) containing white feldspar (labradorite).

To the north of Gundet, it is a green basalt with zeolites, while at Gundet it is a black basalt with a fine paste. However, the general mass of all these basaltic rocks is formed by a true basalt, so that the more or less anomalous basaltic rocks that we have cited are superficial accidents of the basalt. Only they occur on a large scale in Abyssinia.

The tertiary terrain of Enticho is pierced and surmounted by the greenish-black basalt, which constitutes in this locality very high mountains. It is the same in the vicinity of Adigrat. In the vicinity of Guldam, the basalt forms several hillocks and has considerably modified the surrounding rocks. It is sometimes a black, reddish basalt, sometimes a greenish black amygdaloid basalt with zeolite variolas.

This is the seventeenth installment of the geological description of Tigrinyaland² and Semien³, which has been adapted from Messrs. Ferret and Galinier’s work published in 1847.⁴

Reference Notes

1. Abyssinia is a reference to the region which encompasses the modern-day states of Ethiopia, Eritrea, Djibouti, parts of Sudan and parts of Somalia. ↩︎
2. 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. ↩︎
3. 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. ↩︎
4. 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. ↩︎