lOI 



NATURE 



\_Dec. 2, i< 



apparently a second and higher plateau. The abrupt 

 change of level, together with the alteration in the 

 internal structure and the presence of intrusive rocks at 

 the base of the mountain seem to point to the existence 

 of a fault of considerable magnitude, which probably is 

 the eastward extension of a great fault to be described 

 further on. 



The rocks composing this high tract of country consist 

 mainly of clay-slates with the original bedding still very 

 distinct. What may be their exact relations to the 

 granites which they probably overlie, or to the meta- 

 morphic rocks of the coast-range, we have as yet no 

 means of ascertaining. Careful research will be required 

 before anything definite can be said about them. The 

 mountains cut out of these rocks by denudation are 

 rounded in form, smooth, and by no means picturesque. 

 They are devoid of trees, but covered with grass. 



As we approach Lake Nyassa we observe evidence 

 of much disturbance, till at a distance of about ten miles 

 from the Lake we come upon the ancient pipe of a 

 volcano, and five miles further on enter amongst a series 

 of volcanic porphyrites, tuffs, and agglomerates forming 

 mountains several thousands of feet in height, and which 

 extend round the north end of the Lake. Along with this 

 marked change of internal structure ^ye have as decided 

 a change in the scenery. The rounded mountains with 

 smooth, grassy, and uncut sides give place to jagged 

 peaks, serrated ridges, sharp yawning valleys, and 

 irregular, rocky, notched sides, forming a landscape of 

 no ordinary description. 



The extraordinary series of volcanic rocks which form 

 the magnificent mountains round the north end of Lake 

 Nyassa probably belong to the same period as a similar 

 series which characterise the Cape geology. The latter 

 have been assigned to the Trias, and doubtless the 

 immense development of volcanic rocks in Abyssinia 

 described by Blandford is of the same age. Indeed we 

 might almost say we have connecting links between the 

 two places, as on my return march through Ugogo I 

 observed evidence of volcanic outbursts, and it is well 

 known that Kilimanjaro, further north, is of volcanic 

 origin. It seems then that in Triassic times a great line 

 of volcanic action stretched from the Cape by Nyassa, 

 Ugogo, and Kilimanjaro, to Abyssinia. 



But at the north-west corner of Nyassa we have 

 evidence of later volcanic activity. In a niche cut out of 

 the surrounding plateau and on a comparatively level 

 plain, through which the River Jumbaka winds to the 

 lake, a number of beautifully isolated cones rise to a 

 height of about 300 feet. On examination these prove to 

 be perfect volcanic craters, so entire and symmetrical as 

 to appear almost artificial. One crater which I examined 

 forms a beautifully bowl-shaped hollow, descending to 

 the level of the plain, the bottom being a charming 

 circular pond, where a number of hippopotamuses live. 



It is cleir from the perfect shapes of these cones, and 

 from the fact that the surface features of the surrounding 

 country have remained unchanged since their origin, that 

 they must have arisen in comparatively recent times. 

 Besides these cones there are two pretty circular lakes, 

 which also appear to have been originally volcanic 

 craters. 



On leaving this interesting country and proceeding on 

 our way to Tanganyika we rise once more to the top of 

 the pi iteau, cross over mountains Sooo feet in height, and 

 then descend to a general level of from 4000 to 6000 feet. 

 We pass over clay slates and schists whose relative posi- 

 tions could not be determined with intrusive masses of 

 granite. At one point an interesting section was revealed, 

 showing the granite completely inclosing a mass of 

 greenstone. 



On nearing the south end of Lake Tanganyika we pass 

 abruptly from these ancient rocks to red and variegated 

 sandstones much hardened and broken, bu'; preserving 



their original horizontal bedding. Rounding the end of 

 the lake and continuing our march northward along its 

 western side, we come to almost a sheer precipice, sud- 

 denly lowering the altitude from nearly 5000 feet to less 

 than 3000. Running east and west along the precipice 

 there occur intruded rocks, while on the northern or 

 lower side of the precipice the sandstones almost dis- 

 appear, being only represented by a small extent of 

 crushed and tilted beds. Such a condition of things 

 clearly indicates the existence of a great fault. This 

 theory is strengthened by a similar abrupt change of 

 rocks on the eastern side of the lake ; and it will be 

 remembered that we have already noticed among a 

 different series of rocks still further east a sudden change 

 of level almost on the same parallel of latitude. 



The sandstones thus abruptly brought to a finish in 

 their extension northward are succeeded by felspathic 

 rocks which form huge mountain masses both on the east 

 and west sides of the lake. Near the middle of the lake 

 on its western side there occurs a curious apparently 

 isolated area of fine red sandstones, surrounded on all 

 sides except the east by mountains of metamorphic and 

 felspathic rocks. These sandstones would seem to have 

 been deposited in a small lake eight miles in diameter. 

 Mount Malumbi, figured in Stanley's "Dark Continent," 

 belongs to the same formation. 



Still proceeding along the lake we cross a high mountain 

 range named Tchansa, formed of metamorphic rocks with 

 felspathic rock in the centre. We regain the sandstones 

 once more in the country of Uguha. The sandstones 

 here, unlike those of the south end, are very red in colour, 

 extreinely friable, and marked by the abundance of quartz 

 pebbles. Through this formation the Lukuga River 

 finds its way to the Congo, its course determined not by 

 any great convulsion as some travellers have been inclined 

 to believe, but by the long-continued action of streams 

 wearing down the soft and friable barrier which hemmed 

 in the lake at this point. These sandstones have an 

 extension over a large area. They are found away 

 towards Manyema and up the Congo V'alley as far as 

 Lake Moero, probably turning round and joining the 

 strata we have noticed at the south end of Tanganyika. 

 On the east side they are found from Kaboga to the north 

 of Ujiji, though here shales are not uncommon and the 

 strata much curved. 



The absence of all fossils leaves the question of the 

 age of these rocks in some mystery. A reference to Cape 

 geology may, however, as in the case of the volcanic 

 rocks, throw some light on this subject. The Tanganyika 

 sandstones have evidently been formed in an enormous 

 inland lake, beside which the present African lakes 

 would look insignificant. 



In Cape Colony a similar series of rocks occur of a 

 lacustrine origin, and which have been assigned to a 

 period not later than the Trias, and probably they belong 

 to Pateozoic times. In the absence of anything but 

 lithological evidence we cannot do better than place the 

 Tanganyika sandstones in the same era as the Cape 

 series, an era which would seem to have been emphatically 

 characterised by the presence of great lakes. 



Joseph Thomson 



INCANDESCENT ELECTRIC LIGHTS 



THE recent experiments of Mr. J. W. Swan of New- 

 castle-on-Tyne have gone far towards demonstrating 

 the practicabilty of a system of electric lighting based 

 upon the so-called principle of incandescence. As the 

 solution of the whole question of the possible domestic 

 application of electric lighting depends in all probabdity 

 uppon the successful application of this method, these 

 experiments have claimed already a considerable share 

 of public attention, though no panic has yet arisen like 



