ADEN PENINSULA. 



Numerous dykes, varying- greatly in composition and thickness, 

 intersect the rocks, and are of interest in connection with the water 

 question, as are also the slips or small faults occasionally met with. 

 Secondary and accidental minerals are not numerous; chalcedony is 

 common lining cavities in the rock, and thin seams of epidote also 

 occur. In one place I observed a thin seam of gypsum, and close to it 

 under an overhanging rock, a white efHoreseence of common salt. 



Now, with regard to the water-bearing properties of these different 



, , . , . rocks, there are masses of compact lava, which, if 



Artesian borings. ■■ ^ } 



free from fissures or joints, would no doubt be very 

 nearly impervious to water ;' but, as far as I have observed, more or less 

 jointing and Assuring is always present, as seen in the stone quarries above 

 the site of the proposed new bund. Even so however a considerable degree 

 of imperviosity might perhaps be looked for in some of these beds, and for 

 water-bearing strata, we have the porous vesicular lavas. These, however 

 it must be remembered, can only be looked for in their most porous 

 state near the top of the lava flows, as they must necessarily become more 

 compact lower down, and therefore it is extremely improbable that a 

 highly pervious lava bed of any great thickness would be found. As far 

 as I have seen, the tufa beds are too small and local to be of any service. 

 For an artesian boring, therefore, the most hopeful conditions to be expect- 

 ed are a bed of vesicular lava passing downwards into compact, but 

 more or less jointed and fissured rock, and covered by similar compact 

 rock of a more recent flow, and that such bed of considerable thickness 

 and superficial extent could be found is by no means likely. The question 

 would also be rendered more uncertain by the possible existence of dykes 

 cutting through both porous and impervious beds, and of faults brinoino- 

 one into contact with the other. Then with respect to the lie of such 

 beds ; in this as in other volcanoes, the strata have, taken as a whole, a 

 quaquaversal inclination from the edge of the crater (there being however 

 local exceptions where they have apparently been subject to disturbance 



( 263 ) 



