Tertiary Volcanic Terrains of Tigrinyaland and Semien: Trachytes

The rocks of igneous origin which belong to the tertiary terrain are trachytes, basalts, basanites, retinites, amygdaloids, etc. However, all these rocks can be grouped into two categories: trachytes and basalts.

The outflow of trachytes and basalts has considerably changed the relief of Abyssinia¹ since the formation of the tertiary terrains. Generally these rocks form rounded hillocks or cones above the plateaus and mountains. These rocks appear on a large scale in the provinces of Tigray,² Shiré and Semien³. They give rise to the highest points of Abyssinia.

Trachytes

Trachytes are more or less crystalline rocks, composed essentially of rhyacolite. We have lumped together with the trachytes all other rocks which have a similar normal composition and which play the same geological role.

The trachytes seem to be circumscribed between Mereb and Adwa for Tigray; in Tsimbila for Shiré; and, between Qollawoggera and Woggera for Semien. Ordinarily they are found in a lower position than the basalts, and the latter appear to have crossed them.

In Aradeb, we see a rough, cellular and amphiboliferous trachyte. Between Ghendebta and Adwa, it is sometimes a purplish trachyte, sometimes a yellowish trachyte. These trachytes usually contain various minerals, such as amphibole, titanium iron, etc. They are sometimes more or less cellular and often altered.

At Adi-Qesti, we see a bluish gray granitoid trachyte. In the Semien, we often find, closely related to the trachytes, a retinite with small crystals of hyaline quartz with disseminated hyalite or opal. But this retinite has the same normal composition as the trachyte, and plays the same role.

This is the sixteenth 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. 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. 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. 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. 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. ↩︎