Intermediate Lands of Tigrinyaland and Semien: Phyllitic Terrains and Intermediate Talcschist

This is the fourth 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 article, the intermediate or transitional terrains of Tigrinyaland and Semien composed of phyllitic terrains and intermediate talcschist rocks are discussed.

The lands we have defined as intermediate comprise two terrains:⁴

1. The lower or phyllitic terrain; and
2. The upper or graywackic terrain.

In Abyssinia,⁵ the intermediate lands are very developed, and even form the basis of the soil in the center of the country. They are found at extremely variable heights, but they generally form the floor of the valleys. They give rise to a flat, undulating or jagged ground, which is often very picturesque. The view of the mountains around Adwa gives an idea of the configuration of the soil formed by these terrains.

The most fertile parts of Abyssinia are usually in the regions formed by the intermediate lands. Despite the great development of these terrains, however, we were only able to recognize one part of them, the lower part which we relate to the phyllitic terrain. It is probable that the second part, that is to say the graywackic terrain, exists in regions that we have not explored, or that this terrain is hidden by the tertiary terrains in the area that we have explored. With all the more reason, as the series of terrains is almost complete in this part of the globe, and since we found a eurite which probably relates to the graywackic terrain. In any case, we saw no fossils in these intermediate terrains, which demonstrates that if it exists, it only exists in small quantities.

Phyllitic Terrains

In Abyssinia, the phyllitic terrain, or the lower part of the intermediate terrains, is formed of a fairly large number of rocks, among which are found talcschist, phyllite, limestone, calcschist, graywacke, sandstone, quartzite, syenite, pegmatite, etc. However, among the stratified rocks, phyllite plays the main role while among the unstratified rocks, it is syenite. If we take the average direction of the mountains and valleys which are formed mainly of the rocks of the phyllitic terrain, and the average direction either of the layers or of the masses and veins composed of these rocks, we find that the general direction of the phyllitic terrain is from northeast to southwest, that is to say parallel to the coast of the Gulf of Aden, in the Indian Ocean. While the general direction of the gneissic terrain takes place parallel to the Red sea, as stated before.

Intermediate Talcschist

We have discussed the composition of talcschist in the previous part of this article. But, in Abyssinia, as in many other countries, the talcschist of primary lands and the talcschist of intermediate lands, although having the same normal composition, generally differ from each other by some external characteristics, their respective appearances and by their associations with other rocks. Talcschist from primary lands sometimes passes to micachist and protogine by its physical characteristics, by its association with these two rocks and even by its composition; while the talcschist of intermediate lands passes into phyllite, to which it is, moreover, subordinate in Abyssinia.

Thus the talcschist of the intermediate terrains of Abyssinia forms a rock subordinate to the phyllite rather than an independent rock. But it generally occupies the lower part when these two rocks are united. The talcschists that we reported previously as belonging to the intermediate lands are those of the valley of Giba and the surrounding area, those of Digsa, Logo-Sarda, Adwa, and those of Tekezé and the valleys which lead to this great river. (We have already given the description of the talcschists of Giba, when speaking of the primary lands. We will therefore not repeat this description here, although it would have been better placed in this section.) In describing the phyllitic rocks of Digsa, Logo-Sarda and Adwa, we will go into some detail about the talcschists of these localities. With regard to the talcschists of the Tekezé valley and the valleys leading to this river, we will add a few explanatory words.

Talcschists appear exposed around Tukul at the edge of the Tertiary plateau and below the phyllites. In the upper part, it is a pink talcschist, or a greenish-gray talcschist passing to slate or graywacke, or a yellow talcschist passing to phyllite. In the lower part, it is on the contrary a pink garnetiferous talcschist and passing to micaschist. These talcschists are covered, near Tukul, by a small isolated Tertiary plateau. Moreover, they are crossed by diorites, causing the most varied accidents. But they are generally directed from the northeast to the southwest. According to the relative positions of these talcschists, the phyllites and the rocks of igneous origin, it is very probable that the first rocks result from more or less great modifications that the phyllites and graywackes have undergone.

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