R. R. Walls — TJie Geology of Portuguese Nyasalaiul. 207 



the south-east of the country at Jogula and Pomune, where the 

 east-west folds of the Lurio valley meet the north-south folds of the 

 two anticlinal ridges which run parallel with the coast. It is found, 

 also, in the Medo region of the interior, where the east-west folds of 

 gneiss again meet folds at right angles more or less. These 

 latter Inselberge represent areas of extieme compression, and the 

 rocks have been broken and thrown up. All the boulders are 

 severely contorted, and show signs of the intense strain to which they 

 have been subjected. This type of mountain is naturally not smooth 

 in outline like the former three types, but broken and rent in all 

 directions. Huge cavities are found in them, generally the retreat 

 of wild animals. There are extensive footholds for soils, and spiny 

 jjrickly plants grow all over them with such natural entanglement 

 that it is well-nigh impossible to force a passage to the summit. 



But many of the folded whaleback formations of the interior 

 have also snapped and broken by extreme compression, and when 

 this has happened the smooth contour is naturally destroyed . These 

 faults from their appearance are all of very recent date, and are 

 marked by sharp edges. Plants grow and spread rapidly in the debris 

 along these cracks and the action of weathering then tends to split 

 them into folia which run parallel with the long axis of the formation. 

 Although rapidly splitting up, however, they still show their original 

 whaleback character. 



The weathering of the rocks in this country also deserves notice. 

 Owing to the shortness of the wet season, water plays a very small 

 part in the denudation of the country. The chief weathering agent 

 is insolation, the exj^ansion and contraction of the rock due to 

 alternate heat and cold, and in tropical countries this is no incon- 

 siderable factor. Numerous travellers have commented on the 

 sharp crackling sounds given oQ by the rocks in disintegrating 

 by this agency. Insolation causes the rocks to weather in a scaly 

 fashion, breaking oft" layer by layer as it were, and leaving tlie smooth 

 rounded surface alieady commented on in tlte description of the first 

 three types of Inselberge. On many of tJiese loselberge the contour 

 was so smootli that it was impossible to break off a specimen with 

 a geological hammer, and the exact nature of the gneiss could scarcely 

 be ascertained by a cursory examination of the weathered si^rface. 

 The products of disintegration give rise to a coarse, sandy soil, 

 which accumulates in the valleys at such a rate that the rivers in 

 their short season cannot remove it. All the river valleys are choked 

 with sand and after a season of perhaps two months the rivers 

 sink down and disappear in the sand in their own courses. So the 

 corrugated mountainous nature of the country is being obliterated 

 by the products of its own disintegration. Coarse sand is filling 

 up the synclines and valleys and smoothing out the contours, and 

 only the tops of isolated mountains now appear above the general 

 level. These Inselberge may now be submerged in the sandy soil 

 all but a few inches, in which case they are noticeable only by ths 



