The Gash or Mareb is the most northerly of the Abyssinian
rivers which flow towards the Nile valley. Its head-waters
rise on the landward side of the eastern escarpment within
50 miles of Annesley Bay on the Red Sea. It reaches the
Sudan plains near Kassala, beyond which place its waters are
dissipated in the sandy soil. The Mareb is dry for a great
part of the year, but like the Takazze is subject to sudden
freshets during the rains. Only the left bank of the upper
course of the river is in Abyssinian territory, the Mareb
here forming the boundary between Eritrea and Abyssinia.
(3) The Abai---that is, the upper course of the Blue Nile--has
its source near Mount Denguiza in the Goiam highlands (about
11 deg. N. and 37 deg. E.), and first flows for 70 m. nearly due
north to the south side of Lake Tsana. Tsana (q.v.), which
stands from 2500 to 3000 ft. below the normal level of the
plateau, has somewhat the aspect of a flooded crater. It has
an area of about 1100 sq. m., and a depth in some parts of 250
ft. At the south-east corner the rim of the crater is, as
it were. breached by a deep crevasse through which the Abai
escapes, and here dovelb. ps a great semicircular bend like that
of the Takazzo, but in the reverse direction---east, south
and north-west---down to the plains of Sennar, where it takes
the name of Bahr-el-Azrak or Blue Nile. The Abai has many
tributaries. Of these the Bashilo rises near Magdala and
drains eastern Amhara; the Jamma rises near Ankober and drains
northern Shoa; the Muger rises near Adis Ababa and drains
south-western Shoa; the Didessa, the largest of the Abai's
affluents, rises in the Kaffa hills and has a generally S. to
N. course; the Yabus runs near the western edge of the plateau
escarpment. All these are perennial rivers. The right-hand
tributaries, rising mostly on the western sides of the
plateau, have steep slopes and are generally torrential in
character. The Bolassa, however, is perennial, and the
Rahad and Dinder are important rivers in flood-time.
In the mountains and plateaus of Kaffa and Galla in the
south-west of Abyssinia rise the Baro, Gelo, Akobo and
other of the chief affluents of the Sobat tributary of the
Nile. The Akobo, in about 7 deg. 50' N. and 33 deg. E., joins the
Pibor, which in about 8 1/2 deg. N. and 33 deg. 20' E. unites with
the Baro, the river below the confluence taking the name of
Sobat. These rivers descend from the mountains in great
falls, and like the other Abyssinian streams are unnavigable in
their upper courses. The Baro on reaching the plain becomes,
however, a navigable stream affording an open waterway to the
Nile. The Baro, Pibor and Akobo form for 250 m. the W. and
S.W. frontiers of Abyssinia (see NILE, SOBAT and SUDAN.)
The chief river of Abyssinia flowing east is the Hawash
(Awash, Awasi), which rises in the Shoan uplands and makes
a semicircular bend first S.E. and then N.E. It reaches the
Afar (Danakil) lowlands through a broad breach in the eastern
escarpment of the plateau, beyond which it is joined on its
left bank by its chief affluent, the Germama (Kasam), and
then trends round in the direction of Tajura Bay. Here the
Hawash is a copious stream nearly 200 ft. wide and 4 ft.
deep, even in the dry season, and during the floods rising
50 or 60 ft. above low-water mark, thus inundating the plains
for many miles along both its banks. Yet it fails to reach
the coast, and after . a winding course of about 500 m.
passes (in its lower reaches) through a series of badds
(lagoons) to Lake Aussa, some 60 or 70 m. from the head.of
Tajura Bay. In this lake the river is lost. This remarkable
phenomenon is explained by the position of Aussa in the
centre of a saline lacustrine depression several hundred
feet below sea-level. While most of the other lagoons are
highly saline, with thick incrustations of salt round their
margins, Aussa remains fresh throughout the year, owing to
the great body of water discharged into it by the Hawash.
Another lacustrine region extends from the Shoa heights
south-west to the Samburu (Lake Rudolf) depression.
In this chain of lovely upland lakes, some fresh, some
brackish, some completely closed, others connected by short
channels, the chief links in their order from north to south
are:---Zwai, communicating southwards with Hara and Lamina,
all in the Arusi Galla territory; then Abai with an outlet
to a smaller tarn in the romantic Baroda and Gamo districts,
skirted on the west sides by grassy slopes and wooded ranges
from 6000 to nearly 9000 ft. high; lastly, in the Asille
country, Lake Stefanie, the Chuwaha of the natives, completely
closed and falling to a level of about 1800 ft. above the
sea. To the same system obviously belongs the neighbouring
Lake Rudolf (q.v.), which is larger than all the rest put
together. This lake receives at its northern end the waters
of the ()mo, which rises in the Shoa highlands and is a
perennial river with many affluents. In its course of some
370 m. it has a total fall of about 6000 ft. (from 7600
at its source to 1600 at lake-level), and is consequently
a very rapid stream, being broken by the Kokobi and other
falls, and navigable only for a short distance above its
mouth. The chief rivers of Somaliland (q.v.), the Webi
Shebeli and the Juba (q.v.), have their rise on the
south-eastenn slopes of the Abyssinian escarpment, and the
greater part of their course is through territory belonging to
Abyssinia. There are numerous hot springs in Abyssinia, and
earthquakes, though of no great severity, are not uncommon.
(4) Geology.----The East African tableland is continued
into Abyssinia. Since the visit of W. T. Blanford in
1870 the geology has received little attention from
travellers. The following formations are represented:--
Sedimentary and Metamorphic.
Recent. Coral, alluvium, sand.
Tertiary. (?) Limestones of Harrar.
Jurassic. Antalo Limestones.
Triassic (?). Adigrat Sandstones.
Archaean. Gneisses, schists, slaty rocks.
Igneous.
Recent. Aden Volcanic Series.
Tertiary, Cretaceous (?). Magdala group.
Jurassic. Ashangi group.
Archaean.--The metamorphic rocks compose the main mass
of the tableland, and are exposed in every deep valley
in Tigre and along the valley of the Blue Nile. Mica
schists form the prevalent rocks. Hornblende schist
also occur and a compact felspathic rock in the Suris
defile. The foliae of the schists strike north and south.
Triassic (?).---In the region of Adigrat the metamorphic
rocks are invariably overlain by white and brown sandstones,
unfossiliferous, and attaining a maximum thickness of 1000
feet. They are overlain by the fossiliferous limestones of
the Antalo group. Around Chelga and Adigrat coal-bearing beds
occur, which Blanford suggests may be of the same age as the
coal-bearing strata of India. The Adigrat Sandstone possibly
represents some portion of the Karroo formation of South Africa.
Jurassic.---The fossiliferous limestones of Antalo are
generally horizontal, but are in places much disturbed
when interstratified with trap rocks. The fossils are
all characteristic Oolite forms and include species of
Hemicidaris, Pholadomya, Ceromya, Trigonia and Alaria.
Igneous Rocks.---Above a height of 8000 ft. the country consists
of bedded traps belonging to two distinct and unconformable
groups. The lower (Ashangi group) consists of basalts and
dolerites often amygdaloidal. Their relation to the Antalo
limestones is uncertain, but Blanford considers them to be
not later in age than the Oolite. The upper (Magdala group)
contains much trachytic rock of considerable thickness,
lying perfectly horizontally, and giving rise to a series of
terraced ridges characteristic of central Abyssinia. They are
interbedded with unfossiliferous sandstones and shales. Of
more recent date (probably Tertiary) are some igneous rocks,
rich in alkalis, occurring in certain localities in southern
Abyssinia. Of still more recent date are the basalts and
ashes west of Massawa and around Annesley Bay and known as
the Aden Volcanic Series. With regard to the older igneous
rocks, the enormous amount they have suffered from denudation
is a prominent feature. They have been worn into deep and
narrow ravines, sometimes to a depth of 3000 to 4000 ft.
(5) Climate.---The climate of Abyssinia and its dependent
territories varies greatly. Somaliland and the Danakil lowlands
have a hot, dry climate producing semi-desert conditions; the
country in the lower basin of the Sobat is hot, swampy and
malarious. But over the greater part of Abyssinia as well
as the Galla highlands the climate is very healthy and
temperate. The country lies wholly within the tropics, but
its nearness to the equator is counterbalanced by the elevation
of the land. In the deep valleys of the Takazze and Abai,
and generally in places below 4000 ft., the conditions are
tropical and fevers are prevalent. On the uplands, however,
the air is cool and bracing in summer, and in winter very
bleak. The mean range of temperature is between 60 deg. and
80 deg. F. On the higher mountains the climate is Alpine in
character. The atmosphere on the plateaus is exceedingly
clear, so that objects are easily recognizable at great
distances. In addition to the variation in climate dependent
on elevation, the year may be divided into three seasons.
Winter, or the cold season, lasts from October to February,
and is followed by a dry hot period, which about the middle of
June gives place to the rainy season. The rain is heaviest in
the Takazze basin in July and August. In the more southern
districts of Gojam and Wallega heavy rains continue till
the middle of September, and occasionally October is a wet
month. There are also spring and winter rains; indeed rain
often falls in every month of the year. But the rainy season
proper, caused by the south-west monsoon, lasts from June to
mid-September, and commencing in the north moves southward.
In the region of the Sobat sources the rains begin earlier
and last longer. The rainfall varies from about 30 in. a
year in Tigre and Amhara to over 40 in. in parts of Galla
land. The rainy season is of great importance not only to
Abyssinia but to the countries of the Nile valley, as the
prosperity of the eastern Sudan and Egypt is largely dependent
upon the rainfall. A season of light rain may be sufficient
for the needs of Abyssinia, but there is little surplus water
to find its way to the Nile; and a shortness of rain means
a low Nile, as practically all the flood water of that river
is derived from the Abyssinian tributaries (see NILE.)
(6) Flora and Fauna.--As in a day's journey the traveller
may pass from tropical to almost Alpine conditions of
climate, so great also is the range of the flora and fauna.
In the valleys and lowlands the vegetation is dense, but
the general appearance of the plateaus is of a comparatively
bare country with trees and bushes thinly scattered over
it. The glens and ravines on the hillside are often thickly
wooded, and offer a delightful contrast to the open downs.
These conditions are particularly characteristic of the northern
regions; in the south the vegetation on the uplands is more
luxuriant. Among the many varieties of trees and plants
found are the date palm, mimosa, wild olive, giant sycamores,
junipers and laurels, the myrrh and Other gum trees (gnarled
