10 F, P. Mennelt — Basic Dykes and Rock Genesis. 



seas were again separated as we find them to-day. The Nile mouth 

 gradually progressed northwards as the land rose, the river cutting 

 its present channel in the sands which it had itself previously 

 deposited ; and when the Isthmus of Suez was just about to emerge 

 from the sea, a part of the river may well have discharged itself 

 eastwards towards what was then the junction of the two seas, thus 

 accounting for the alluvial deposits and Nile shells which occur on 

 the isthmus. 



There is some evidence that the upward movement of land in 

 North-Eastern Egypt which went on in later Pleistocene times has 

 now ceased, and given place to a slow sinking. Lyell 1 mentions the 

 positions of tombs near Alexandria and the ruins of towns half 

 submerged in Lake Menzala as evidence of this, and lately Mr. Yilliers 

 Stuart 2 has observed that certain areas of the northern delta, which 

 carried a dense population in Roman times, are now uninhabited and 

 only a few centimetres above sea-level. 



I venture to think that the outline which I have given above of 

 the later geological history of the Gulf of Suez explains, in a perfectly 

 natural manner, all the facts of observation which it has hitherto 

 been sought to explain on the trough-fault theory, and with less 

 straining of probabilities. Our knowledge of the detailed structure 

 of the gulf is far from being of that completeness which is desirable 

 in a district which may possibly become one of the great oilfields of 

 the world ; but a working hypothesis of some sort as to the origin 

 of the gulf is desirable, and I urge that erosion and secular oscillation 

 is a far better working hypothesis, in the light of presently known 

 facts, than local subsidence by trough-faulting. 



II. — Observations on some Basic Dykes and their Bearing on 



certain Problems op Koch: Genesis. 



By F. P. Mennell, F.G.S. 



(PLATES I AND II.) 



IT is a common feature of the dolerite dykes in Rhodesia, especially 

 where they penetrate the great granite masses of the country, 

 that they show, even in hand-specimens, the presence of quartz. 

 Usually this quartz occurs in good-sized corroded fragments which 

 are quite obviously xenoliths derived from the surrounding rocks. Of 

 this a fine example is to be seen close to Bulawayo Railway Station, 

 where a small dyke is at one point so full of quartz fragments, up to 

 several inches across, as to resemble a conglomerate more than an 

 igneous rock. Other parts of this same dyke and many other basic 

 dykes throughout the country contain quartz, either in separate 

 granules or in the form of micropegmatite, whose origin is not so 

 clear. It is with a view to elucidating this point that the present 

 notes have been put together. They deal entirely with two intrusions 

 cutting the great Matopo granite mass. The relative ages of the 

 dykes and the granite cannot be given in precise terms, as the data 



1 Principles of Geology, 10th ed., vol. i, p. 438, 1S67. 



2 Cairo Scientific Journal, 1909, p. 230. 



