Primary Lands of Tigrinyaland and Semien: Granite, Gneiss and Related Rocks

This is part 2 of the geological description of Tigrinyaland¹ and Semien² which has been adapted from Messrs. Ferret and Galinier’s work published in 1847.³ Today we will discuss the primary terrains of Tigrinyaland and Semien which are composed of granite, gneiss and related rocks as reported by the two French explorers.

Primary terrains are made up of the oldest rocks that are related to each other. They include granite, gneiss, micaschist, schistose protogine, part of the talcschists and various other rocks which are subordinate to those we have just listed. All of these probably belong to the gneissic terrain of Mr. Rivière.⁴ If we take the direction of the Tarenta mountains,⁵ which are formed mainly of primary rocks of the gneissic terrain and the average direction of the various rocks seen by us that will be described below, we find that the general direction of the primary lands of Abyssinia takes place substantially from the northwest to the southeast. Now we will proceed to our description of the rocks of the primary terrains observed by us in Abyssinia, according to the order of superposition or, in other words, according to the most probable chronological order of formation.

Granite

Granite is a crystalline rock essentially composed of orthoclase, quartz and mica. We find granite on the road from Halay to Goul-zobo, about 15 kilometers northeast of the latter place. It is usually a whitish or grayish granite.

Between Gundet and Hadish-Adi, the granite forms the bottom of the valley in which the Mereb flows. It is a gray syenitic granite, almost without quartz, with black-green mica and often amphibole. It is surrounded by syenite and crossed by veins of diorite, circumstances which must have modified it more or less. This granite weathers easily and forms a sandy bed on which the Mereb rolls its waters.

From Hadish-Adi to Aderba’ite, we see the granite in the bottom of several valleys, where it is sometimes exposed and sometimes covered by other rocks and mainly by phyllite. This granite, which is probably related to that of the banks of the Mereb, is usually whitish or grayish.

Pegmatite, which is a crystalline rock ordinarily subordinate to granite, and which is essentially composed of orthoclase and quartz, occurs in a few localities. We will cite the one we found in a ravine located between Adi-Bahra and Firfira, as well as that seen in the Merakat ravine, Shiré plain; but, in both cases, the pegmatite, which forms the bottom of these ravines, seems to be associated rather with syenite than with granite. It remains to be seen whether the pegmatite crosses the syenite and therefore forms a rock independent of it, a circumstance which would give it a more modern age than the appearance of the syenite and much less ancient than the formation of the rocks of the gneissic terrain. However, the pegmatite in the ravines of Merakat and Firfira appears to be closely linked to the syenite, and to be only a mineralogical accident of the latter rock through the loss of its amphibole.

We have not seen true granite anywhere else; However, it is probable that it appears in other localities, notably in the country of the Sahos; but what is certain is that granite is a rock which rarely appears in Abyssinia, and which consequently is, with few exceptions, hidden everywhere by other rocks.

We are therefore led to regard the pegmatite of Abyssinia as depending rather on syenite than on granite, contrary to the usual fact. We thus link it to the transitional terrains. This syenitic pegmatite of which we are speaking, is generally pink, passes to kaolin through its decomposition and often contains zircon, garnet, etc.

Gneiss

Gneiss is a crystalline rock, more or less schistoid and essentially composed of orthoclase and mica. Gneiss mainly forms two powerful bands in Abyssinia, which are found in the area located between the Red Sea and the Tarenta Mountains. These two bands, which probably form only one below the other rocks, are, like the rest of the gneissic terrain, directed approximately from southeast to northeast. This direction, which is generally that of all the primary lands of Abyssinia, is also the direction of the Tarenta mountain system and the coasts of the Red Sea.

The gneiss field, which is nearest to the Red Sea, is also the lowest. It passes around Sa’átit, between Mitsuwa and Aylet, to form the first mountains that we encounter when entering Abyssinia through Mitsua. This band of gneiss serves as a border to the modern terrains of the Red Sea coasts. In this gneiss, we see mica in large strips of twenty-five to thirty centimeters on a side. The second band of gneiss follows the foot of the last chain of Tarenta. Gneiss probably forms the base on which rests this chain whose highest point is at least two thousand five hundred and forty-three meters above the Red Sea. The first band of gneiss is far from reaching this height, although it rises to more than fourteen hundred meters above the same level. In the Tarenta chain, the gneiss is often garnetiferous, more or less altered, and presents veins of milky-white quartz.

In this part of Abyssinia, gneiss is associated with micaschist, schistose protogines, and appears to be more or less associated with syenite as well as certain talcschists. If gneiss still appears in other parts of Abyssinia, this occurs only rarely and must only occur in the bottom of the valleys.

Reference Notes

1. 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. ↩︎
2. 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. ↩︎
3. 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. ↩︎
4. Rivière, Alphonse (1839) Eléments de Géologie pure et appliquée ou Résumé d’un cours de géologie descriptive, spéculative, industrielle et comparative. Méquignon-Rivière: Paris. ↩︎
5. The Tarenta mountain chain forms the eastern escarpement of modern-day eritrea, and looks down on the Eritrean eastern lowlands which are close to the Red Sea. ↩︎