1835.] 



President's Address. 



287 



any indications of an approach to the solid rock. The samples of 

 the Delta deposits obtained by these boring operations are found to 

 be in all cases mixtures, in varying proportions, of Nile mud, or 

 material carried in suspension by the river and desert sand, or parti- 

 cles swept in from the surrounding districts by the action of winds . 

 The study of these materials by the aid of the microscope has 

 revealed a number of facts which may be made the basis of generali- 

 sations of considerable interest to geologists. 



The minerals present in these sands and muds are found to be such 

 as characterise the granitic, and highly crystalline metamorphic rocks ; 

 there can be litle doubt, therefore, that the vast regions included 

 within the Nile basin are in the main composed of rocks belonging to 

 those classes, or of sedimentary deposits derived directly from them. 



Of still greater interest is the fact "that the fragments of felspars 

 and other complex silicates in the Delta deposits exhibit but slight 

 evidence of kaolinisation or other chemical change. This points to 

 the conclusion that, in the rainless districts drained by the Nile, the 

 disintegration of rocks is effected by mechanical rather than by 

 chemical agencies. A very striking confirmation of this conclusion 

 is afforded by the study of the composition of the waters of the 

 Nile, our knowledge of which has been greatly advanced by the recent 

 researches of Dr. C. M. Tidy. In spite of the circumstances that the 

 waters of the Nile must undergo great concentration during its 

 passage of 1,400 miles through regions of exceptional heat and 

 drought, it is found that those waters actually hold in solution little 

 more than one-half the percentage of mineral matter which is present 

 in the river waters of temperate and rainy regions. The chemical 

 disintegration of rocks being so largely due to the action of rain and 

 vegetation, it is not surprising to find that where these agencies are 

 almost entirely absent the rocks exhibit but few signs of chemical 

 change. 



The Krakatoa Committee, which is now rather a large one, con- 

 sisting of thirteen members, has been steadily at work during the 

 year ; and the discussion of the very varied and large mass of data 

 has been undertaken by sub-committees, dealing respectively with 

 the following branches : — 



Geological — including eruption and earthquake phenomena, and 

 the geological features of the distribution of dust and pumice. 



Meteorological (A) — including air waves, sounds, and the geo- 

 graphical distribution of dust and pumice. 



Meteorological (B) — including twilight effects, coronal appear- 

 ances, cloud haze, coloured sun, moon, &c. 



Magnetic and electric phenomena. 



Tidal waves. 



vol. xxxi x. u 



