Micaschist, Protogine, and Talcschist – Primary Lands of Tigrinyaland and Semien

This is the 3rd part of the geological description of Tigrinyaland¹ and Semien,² which has been adapted from Messrs. Ferret and Galinier’s work published in 1847.³ In this installment, the primary lands of Tigrinyaland and Semien formed of micaschist, protogine, and talcschist rocks will be presented.

Micaschist

Micaschist is a crystalline, schistoid rock composed essentially of quartz and mica. The micaschist, which is more or less linked to the gneiss and the schistoid protogin, is even, like the last rock, relegated to the surroundings of the Red Sea, from the Tarenta mountains⁴ to the modern terrains of the coasts. It is based on the gneiss, forms two bands which are contained between that of the last rock and which are separated from each other by immense masses of syenite.

This micaschist sometimes grades to gneiss by an addition of orthoclase, sometimes to hyalomicte⁵ by an excess of quartz. In the vicinity of Aylet, it has a pinkish gray hue and a direction from north-northwest to south-southeast like the gneiss of this country.

Protogine

Protogine is a rock essentially composed of albite, talc or chlorite and quartz. It is more or less schistoid, depending on whether it contains more or less talc or chlorite; on the contrary, it is more or less granitoid, depending on whether it contains more or less quartz. Accordingly, protogine, which contains a lot of talc and no quartz, is quite different in composition and texture from protogine, which contains a lot of quartz and little talc. Thus it would perhaps be necessary to establish a distinction in the protogines, and this measure would probably resolve the discussions relating to stratified protogines for some and non-stratified protogines for others. But we believe we must conform to custom here, despite the vagueness that it creates. We will therefore apply the word protogine to the feldspar and talcose rocks of Abyssinia,⁶ which contain little or no quartz and which are constantly schistoid.

Protogine is one of the fundamental rocks of the Tarenta mountains. It forms two bands which are directed, like these mountains, approximately from the northwest to the southeast. This protogine is based on the gneiss, when the micaschist is missing. The two bands are separated from each other by a feldspathic talcschist, but which can be considered as an extreme variety of the protogine which is the main rock , or at least as a talcschist subordinate to protogine.

The Tarenta protogine is crossed by diorite veins and typhons⁷. It is green when it has not undergone any alteration; but when it is altered, it is white and turns to kaolin. The Tarenta protogine is schistoid and forms what some geologists call talcose gneiss or schistose protogine.

This rock constitutes the major part of the mountain, as it appears about 300 meters in elevation above the foot of Tarenta, as well as at 222 meters higher and at the crossing point or pass which has an altitude of 2,538 meters. We also find in Abyssinia other rocks which, mineralogically, could be classified as protogines. But these rocks are only accidents which belong to the talcschists of which we are going to speak.

Talcschist

Talcschist is a schistose rock which is essentially composed of talc and quartz. Talcschist, which is superimposed on micachist or schistose protogine, occurs in various parts of Abyssinia. It sometimes forms considerable areas of land, but it does not always present itself with clearly defined characters, and often only results from the more or less great modification of other rocks.

We should therefore only speak now of the talcschists which appear to belong to the primary lands and the gneissic terrain of Mr. Rivière.⁸ In this category we place the talcschists of Tarenta. Those of the valley of Giba, those of the north of Adwa and those of the Tekezé valley more likely belong to intermediate lands.

When one comes from the Red Sea and climbs the Tarenta mountains, one sees a band of talcschist, which is contained between two bands of schistose protogine and of which it appears to be a dependency or vice versa. This is because the talcschist is feldspathic, amygdalar, green like protogine when the latter is not altered, and passes to this last rock. It is also linked to the protogine by its stratification, and is, like it, crossed by dioritic rocks. Thus, from a geological point of view, we should have linked the description of the talcschist of Tarenta to that of the protogine of the same locality.

Let’s move on to the talcschist of the Giba valley, which we saw on our way from Adigrat to Chelekot. After leaving at Lat a plateau formed of tertiary sandstone, of which we will speak later, we soon arrive at Belesa, where there is a small river that flows into the Giba, whose bed is formed of talcschist. This rock constitutes the soil of all the valleys which are linked to the Giba valley up to the surroundings of Tekezé. But in high places it is crowned by approximately horizontal tertiary deposits. The layers of this talcschist, which are upset, torn and almost vertical in many places, offer a striking contrast with the horizontal and regular appearance of the sandstones and tertiary clays of the plateaus.

In the Tekezé valley, between this river and the Shiré plateau, the talcschist forms the base of the visible soil, gives rise to very picturesque accidents, affects all kinds of inclinations. But its general direction is approximately from northeast to southwest. According to this difference in direction which exists between the talcschists of Tekezé and those of Tarenta, we consider the former as less ancient than the latter. Consequently, we have removed the talcschists of the Tekezé valley, those of the surroundings of Digsa, Logo-Sarda, Adwa, Giba , etc from the gneissic terrain.

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. The Tarenta mountain range forms the eastern escarpement of modern-day eritrea, and looks down on the Eritrean eastern lowlands which are close to the Red Sea. ↩︎
5. Hyalomicte is an obsolete term for a greisen consisting of hyaline quartz and mica. ↩︎
6. Abyssinia is a reference to the region which encompasses modern-day Ethiopia, Eritrea, Djibouti, parts of Sudan and parts of Somalia. ↩︎
7. Typhon is a collective French geological term for the plutonic rocks boss, stock and batholith. ↩︎
8. Rivière, Alphonse (1839) Eléments de Géologie pure et appliquée ou Résumé d’un cours de géologie descriptive, spéculative, industrielle et comparative. Méquignon-Rivière: Paris. ↩︎