304 Professor Bayley Balfour [April 20 



however, many perennial streams on the island, especially in the 

 central granitic region, where amongst the hills the most charming 

 bubbling burns dashing over boulders in a series of cascades, or 

 purling gently over a pebbly shingle, make it hard to believe that 

 one is in such proximity to the desert region of Arabia. Few of the 

 perennial streams reach the shore in the dry season — most of them 

 are fiumaras. 



The eastern end of the island is most destitute of water. Here 

 in the dry season are no rivers, and, springs being rare, it is the most 

 arid region. 



The geology of the island is not very, complex. The funda- 

 mental rocks are gneisses, both hornblendic and granitoid, belonging, 

 like those of north-west Scotland and of north-east America, to the 

 earliest Archaean age. These crop out on the hill slopes and in the 

 valleys, but do not as a rule form the exposed higher parts of the 

 island. Through this fundamental mass cut felspathic granites of 

 varying texture, aud containing little besides quartz and fclsj)ar, which 

 form the central nucleus of fantastic peaks, the highest part of the 

 island. Cutting through both the forementioned series we have other 

 granitic rocks, such as minettc, felsite, rhyolite, and also basalt aud 

 diorites, in many places forming large dykes, and in others extensive 

 lava flows. The centres of ejection of these rocks it is impossible 

 now to determine, and possibly many of them, as in the case of the 

 Tertiary volcanic rocks of East Hindostan, have been discharged, not 

 from cones, but as outflows from fissures. Towards the south-east 

 end of the island we find them in greatest abundance, and exhibiting 

 a very fluidal character. The date of the eruption of these rocks was 

 certainly pre-miocene. An indurated shale (argillite) is found in 

 some localities, notably on Hadibu plain, and with it a little sand- 

 stone of uncertain date, but probably representing the well-known 

 Nubian sandstone of carboniferous age. Over all comes a capping of 

 limestone, forming plateaux over wide areas, rising in abrupt cliffs two 

 or three hundred feet high. It is generally of a yellowish or whitish 

 colour, compact, and sometimes slightly dolomitised. It contains 

 numerous foraminifera, which prove it to be probably of Middle 

 Tertiary age, or rather later than that of Sinai and the Arabian shores 

 of the Ked Sea. The surface of the limestone over extensive districts 

 is rotted and broken into a jagged surface, over which progression 

 is by no means easy, whilst at other spots it forms broad smooth 

 slabs. Subsequent to the laying down of the limestone there occurred 

 further volcanic disturbance, and the limestone is cut through by 

 dykes of basalt and compact trachyte of late Tertiary age. 



The soil resulting from such petrological conditions is corre- 

 spondingly varied, correlated with which is a varying character in 

 vegetation and scenery. 



In the valleys on the banks of the stream, especially in the granitic 

 region, a deep rich red soil is found, and where there is water 

 perennially it is covered by a luxuriant growth. As the limestone 



