712 TRANSACTIONS OF SECTION C. 



effect on them of the thrust, here described, to be observed. The limestone, 

 although much disturbed, has a general low dip to the south. It consists of a 

 series of rocks of different flexibility, rigidity, brittleness, and compressive strength, 

 and the uniqueness of the section is due to the action of a thrust fi'om the north, 

 which has caused some of the beds to move laterally upon the others, with an 

 associated production of folding, minor-thrusting, and Assuring, and a consequent 

 development of dynamic brecciation. 



In Frenchman's Bay the Lower Limestone (40 feet thick) is a brownish- 

 yellow, regularly-bedded rock of relatively high compressive strength and rigidity. 

 Its lower layers are gently folded, but its top layers are considerably disturbed, 

 being fractured, tilted-up, and laterally displaced. Resting on this is about 

 60 feet of Brecciated Limestone, consisting almost entirely of a cemented mass of 

 broken fragment, which have here and there been dissolved out from the 

 cementing matrix, the rock becoming cellular. In places the bedding, unfolded 

 but fractured, is still preserved. It was originally a finely laminated granular 

 limestone, and is of low rigidity and compressive strength. Experiments per- 

 formed by Dr. Morrow give the following results : Compressive strengths (tons 

 per square inch to break rock), Lower Limestone, 6-7 tons; Brecciated Bed (lower 

 middle), 1 ton. 



The junction of the Brecciated Beds with the Lower Limestone is a thrust plane, 

 which does not always coincide with the line of demarcation between the two 

 strata ; and the disturbance of the upper layers of the latter rock, together with 

 the smashing up of the former, is due to the thrust movement. 



The Brecciated Beds occupy the top of tlie cliff for over a mile. Their upper 

 surface, which is seen at the north end of Marsden Bay, is very irregular and 

 hummocky, the breccia having been forced up into the base of the beds above. 

 These consist of about 200 feet of rock difleriug much in flexibility and compres- 

 sive strength (specimens tested vary from I ton to 3-7 tons per square inch). Tliey 

 have been thrust against a horst, and consequently folding, thrust iii^r, a!id 

 dynamic brecciation has taken place. The flexible beds have been deformed 

 without being much broken, while a harder, more brittle, wedge-.shaped limestone 

 has been highly brecciated. The latter has also had a coarse cleavage structure 

 impressed on it, and jDart of it has been torn oft' and thrust into the beds above. 

 Breccia gashes and vertical Assures filled with breccia, which has fallen into them, 

 occur on both sides of this folded and broken area. The amount of lateral dis- 

 placement at this point is difficult to estimate accurately, but it has probably been 

 about 100 yards, and the experiments indicate that the magnitude of the thrust 

 was about 300 tons per square foot. 



1. ^Veil-water Supply of the North-Eastern Sudan. JJi/ (!. W. (Jkabham. 



In the development of the north-eastern Sudan, water-supplies are of supreme 

 importance, and the Government has already expended large sums of money in 

 sinking wells along the railways and roads. 



Artesian conditions have not been met with, and the supplies are dependent 

 on the rainfall and local geological conditions. There are two distinct rainfall 

 systems; one affects the plains during the summer mouths, while the other rains 

 occur during the winter months, and are confined to the Ked Sea coast. 



The country is formed of crystalline rocks, and on the uneven surface of these 

 rest the Nubian sandstone, &c. The surface deposits vary according to the 

 climate and the underlying rock. In the north the conditions are similar to those 

 described by Mr. Ferrar, but to the south, where the rainfall is greater, the plains 

 are covered by a blanket of ' cotton soil.' This soil is very fine grained and absorbs 

 a great deal of water during the rains. In the dry season the moisture evaporates, 

 and the surface becomes fissured with deep wide cracks. A very thin layer of 

 this soil prevents the rainfall from reaching any underlying deposit, which might 

 otherwise become saturated. 



In the northern area the raii^fall does not exceed four iuches pef annum, except 



