﻿192 THE GEOLOGY OF NYASALAND. [May I9IO. 



masses rising irregularly one above the other. The greatest 

 altitude (9843 feet above sea-level) is attained by one of the 

 Mlanje peaks in Southern Nyasaland. Farther north the Chongoni 

 mountain-group reaches nearly 8000 feet, while a considerable 

 area in the Nyika Plateau varies from 7000 to 8000 feet in alti- 

 tude. Lake Nyasa occupies the bottom of a deep north-and-south 

 trench, bordered on the east and west by the truncated edges of 

 the plateau-regions. These at times drop steeply down below the 

 waters of the lake : but usually the western edge of the latter is 

 fringed by a sandy coastal plain, 3 to 20 miles across. 



Lake JS T yasa has a length of about 300 miles, a maximum breadth 

 of some 45 miles, and a maximum depth of more than 670 feet 

 below sea-level, while the surface of the lake lies at an elevation of 

 1645 feet. 



The lake- trough is continued both north and south of the lake. 

 On the north the valley, with an alluvial floor, rises gradually 

 until it is choked with lavas of recent and perhaps Tertiary age. 

 On the south the lake bifurcates, and drains out through its eastern 

 arm into the Shire Valley. Even at first, the latter is several miles 

 in width, and it widens out very considerably farther down. The 

 gradient of this valley for a long distance does not exceed 1 foot 

 per mile. A large but shallow lake, known as Pamalombe, lies near 

 its head : thig, however, is rapidly dwindling in size, and will 

 soon be completely silted up. Below Liwonde the Shire Yalley 

 swerves westwards to meet the continuation of the western limb of 

 the lake-trough, and for many miles now runs in a wide and deep 

 depression between the Neiio Plateau on the west and the Shire 

 Highlands on the east. Somewhat below Port Herald, and in 

 Portuguese territory, the Shire at last escapes from the mountains, 

 and thenceforth meanders across a flat plain on its way to join the 

 Zambezi near Morambala Mountain, a lofty isolated ridge which 

 can be seen for miles across the grass- and tree-covered river-flats. 

 There are only three lakes in Nyasaland, and two of these have 

 already been mentioned. The third, Lake Shirwa, is about 20 

 miles long, and lies in a shallow depression to the north of the 

 Mlanje Mountains. This lake is very shallow, and has no outlet : 

 its water is distinctly brackish, in contrast with that of Lake ISyasa, 

 which is perfectly fresh. A low but broad ridge of gneiss separates 

 Lake Shirwa from the Shire Valley and Lake Pamalombe. These 

 minor lakes probably represent a local sagging in of the earth's 

 crust, consequent upon the great earth-movements which gave rise 

 to the Nyasa trough. This is a subject which will be dealt with 

 more fully in a subsequent portion of the paper (p. 233). 



III. Geology. 



The rocks of Nyasaland may be broadly divided into (a) a crystal- 

 line series comprising crystalline schists, gneiss, and plutonic 

 intrusions, and (b) a later sedimentary series with which may be 

 included the associated lavas and dykes. 



