This is the ninth 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 secondary lands of Tigrinyaland and Semien consisting of diorite, amphibolite, and related rocks will be examined.
As diorites, amphibolites, etc, appeared in other parts of the world, during and especially after the formation of the carbonic terrain, we will also relate those of Abyssinia4 to the same period, with all the more reason that these diorites, amphibolites, etc, crossed the whole series of terrains older than the carbonic terrain, and that we have nowhere seen these rocks intercalated in those of the oolitic terrain. In Abyssinia, diorites, amphibolites, amphibolite serpentine, lepidolite, etc, form only one group of rocks of the same age, the same appearance and the same geological characteristics, that is to say having played the same role and being only accidents of the main rock, the diorite.
Diorite is a crystalline rock composed mainly of amphibole and albite. Amphibolite is a diorite which does not contain albite or which contains very little of it. The serpentine of Abyssinia, which is not a true serpentine, but which is usually confused with it, like many other green rocks, is of a composition similar to that of diorite. We can therefore take diorite as the type of these amphibolitic rocks.
From these considerations, we will say that the diorites played a very important role in the orographic and geognostic constitution, as well as in the dynamic and metamorphic phenomena of Abyssinia. Moreover, we find, almost at every step, massifs or typhons,5 veins and boutons formed of these rocks. Leaving aside generalities, we will review the main deposits of diorites that we have seen in Abyssinia.
Diorite penetrates the rocks of the Tarenta mountains6 at different points. Towards the foot of the mountains, we see a vein of compact epidote, with veinlets of hyaline quartz which is sometimes crystallized. This lepidolite vein crosses the gneiss and syenite. Besides, it is, according to all appearance, only an extreme accident of the veins of epidotous, garnetiferous and sometimes quartziferous diorite, which we see in these mountains, and of which we notice an example between the talcschist and the protogine, which have been more or less modified by these diorites.
At Mai-Kharassat, in the middle of the phyllites, a dark green and homogeneous diorite is observed. This diorite typhon significantly modified the rock it passed through. The phyllite, in its vicinity, also grades sometimes to phtanite, sometimes to talcschist.
At Assira, the phyllites are crossed and partly covered by a fine, greenish-grey diorite. While in Belessa it is an epidotous green diorite, garnetiferous and talciferous, with a serpentine appearance, which plays the same role in the middle of the graywacke and the phyllite. But here, notably at Logo-Sarda, the Phyllitic rocks have passed into a state of feldspar talcschist, and this probably under the influence of the dioritic rock.
In the Hesta valley, at the foot of Debre-Damo, a black, semi-compact amphibolite appears. This rock has crossed the phyllites which it has modified by making their texture more crystalline.
In the mountain range between the left bank of the Mareb and the left bank of the Unguiya, dioritic rocks play a very important role; because they sometimes present themselves as powerful typhons. Moreover, we see them piercing the phyllites and other rocks at every step. So that in this part of the country, the dioritic rocks form the background of a picture, in the middle of which are scattered, here and there, trachytic rocks, syenites, phyllites, etc.
Thus, between Eggala and Gendebta, we find a fine light blue diorite or a yellowish porphyroid diorite. Near Adwa, it is on the contrary a compact, homogeneous, greenish diorite. In the Samaiata mountain, diorites cut and cover the phyllitic rocks which they have modified considerably. The summit of the Shelloda mountain is formed by amphiboles which have further cut the phyllites, as at Debre-Sina, at Nagah, around Maichanao, Addi-Etmot, Dahro-Tecli and in the bed of the Corsoro. Around Abune-Alef, the diorite pierces the syenite.
Around Gundet, it is, on the contrary, the granite which is crossed by the diorite. West of Aylet, the diorite which cuts the syenite is micaceous porphyroid, with albite crystals.
In Nagah, the amphibolite is green and schistoid. Between this last place and Aradeb, the diorite is very fine, bluish, and grades into eurite. In the ravine which is located between Abune-Alef and Hadish-Addi, the diorite is green. It presents black crystals of amphibole and very small amygdules of albite. At Hadish-Addi, the diorite passes to porphyroid eurite.
In Shiré, dioritic rocks are sometimes exposed around the basalts, or at the bottom of the ravines and in the Tekezé valley. Between Adi-Daero and Mai-Khansseh, the diorite is porphyroid. At Mai-Khansseh, it is also generally porphyritic and green. The amphibolite and diorite cross and torture the phyllite. But here the modifications experienced by the phyllites and the graywacke are very developed and very apparent. Generally, these rocks have taken on a more crystalline texture, become talcschist, and contain garnets. The dioritic rocks of this part of the Tekezé valley are sometimes a bluish black amphibolite, sometimes a white porphyroid diorite, sometimes an epidotous rock or lepidolite, etc.
The dioritic rocks which occur, on a large scale, in the Tekezé valley, are even visible on the other side of the river going towards Gondar. In this part of Abyssinia, they play the same role as in the Tigray Province7 and Shiré.
We believe we must relate to the diorites a greenish-yellow serpentinoid eurite, which is found around Digsa. However, this connection must be treated with great doubt.
We would go further than the results of our observations allow, if we were to specify the general direction or the alignments of the typhons, veins and diorite boutons, that is to say if we tried to indicate the direction of the lines of fractures through which the diorites emerge from the bowels of the earth. We will only say that a large number of typhons seem to go approximately from west-northwest to east-southeast. But, we repeat, there is nothing certain in this regard, because our research on the site is not precise enough to address this important question.
Reference Notes
- 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. ↩︎
- 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. ↩︎
- 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. ↩︎
- Abyssinia is a reference to the region which encompasses the modern-day states of Ethiopia, Eritrea, Djibouti, parts of Sudan and parts of Somalia. ↩︎
- Typhon is a French collective geological term for the plutonic rocks boss, stock and batholith. ↩︎
- 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. ↩︎
- 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. ↩︎