R. R. Walls — The Geology of Portuguese Nyasaland. 209 



however, and it is very doubtful whether any of these minerals 

 occur at all in commercial quantities. 



The chief geological feature in Portuguese Nyasaland is the intense 

 compression and folding to which the whole country has been 

 subjected, resulting for the most part in the formation of ridges and 

 valleys running in a north-south direction. Lake Nyasa itself is 

 supposed to occupy a huge fault-line in this direction. But there 

 have been thrusts from the opposite direction as well, resalting in 

 folds at right angles to the above, as, for example, in the valley of 

 the Lurio and across the centre of the country. These two thrusts 

 are quite distinct from each other, and have each affected many 

 thousands of square miles of country. No traveller has yet mentioned, 

 .so far as the writer is aware, the occurrence of these two great 

 earth thrusts in this country, yet these factors will be of vital 

 importance when the geology of this teiritory comes to be studied in 

 detail. The lines of foliation over the whole country must be 

 plotted and the respective areas affected by these two great earth- 

 thrusts mapped out. Much can then be learned with regard to the 

 physical features of the country and the geological sequence of the 

 rocks. Where the writer marked the boundary lines between the 

 two areas in the south-east portion of the territory, much local 

 contortion was evident and mountainous heaps of broken boulders 

 had been thrown up as already described. To this phenomenon 

 are also due the humps or whaleback formations along the line of 

 the main ridges already discussed. It is not suggested that these 

 two thrusts were contemporaneous. On the contrary, the bulk of 

 the evidence seems to indicate that the thrust which produced the 

 north-south ridges was subsequent to that which produced the east- 

 west ridges of the central and southern areas. Indeed, there are 

 many evidences that the former thrust is of recent geological date, 

 and may, in fact, be still going on at the present day. These 

 evidences are : (1) The raised beaches of the coast which, though 

 they may now be 300 feet above sea-level, are all of recent age ; 

 (2) The fault-lines of the raised beaches, e.g. round Pemba Bay, 

 which are still sharp and distinct, although the escarpment is com- 

 posed of soft unconsolidated mud which is wa.shed out by thousands 

 of tons each rainy season ; (3) the anomaly of the deep water of 

 Pemba Bay in a shelving and silting-up coast ; (4) the numerous 

 rivers which come down from the interior and cut through the two 

 north-south ridges parallel to the coast, and which are therefore 

 older than these ridges ; (5) the cracks occasionally seen where the 

 folded rocks have snapped and which are invariably new and angular 

 in appearance ; (6) the folding of the more recent granites along with 

 the older rocks into which they are intrusive. 



It is only after all the evidences of a great earth-thrust from the 

 east have been studied carefully that the geological formation of 

 Pemba Bay becomes comprehensible, and the anomaly of this deep 

 harbour on a low and shelving coast explamed. Pemba Bay has an 



VOL. LIX.— NO. V. 14 



