befiveen Cairo and Suez. 61 



neighbourhood of volcanic vents these injected fissures or plugs of 

 thermal springs are much more numerous. It is also significant 

 that only in the neighbourhood of these thermal fissures does 

 silicified wood occur in abundance. 



In the foregoing paragraphs it has been indicated that the order 

 of succession of the beds in question is, first, the floating in and 

 depositing of the trees in a shallow estuary or lagoon ; second, the 

 pouring out of the basalt ; and third, the breaking out of thermal 

 springs containing silica in solution which petrified the trees in the 

 sands. It might be maintained that the order of succession is not 

 clearly proved between the basalt and the thermal springs. To this 

 the answer is that it would be impossible for a geyser to coexist 

 with an active volcano, and that if it were prior to the volcano the 

 silica deposited by it would have been dissolved and absorbed by 

 the igneous rock. 



The history of these beds appears to be as follows : — After 

 the deposition of the sands, etc., containing the trees, a series of 

 earth-movements took place which culminated in an outburst of 

 basalt on both sides of the present Nile Valley. When the force 

 of the outbui'st had expended itself, the volcanoes, which had not 

 (as far as one can see between Cairo and Suez) poured out very 

 thick sheets of lava, became quiescent, earth-movements continued 

 which threw the rocks into shallow basins, fractured the volcanic 

 necks, and eventually opened a way for the outpouring of heated 

 silicated waters. These waters issued sometimes undoubtedly in 

 the form of a geyser, although in the majority of cases they came 

 out in the form of hot springs. Over the surface of the basalt they 

 flowed, penetrating the cracks and filling them with chalcedonic 

 silica, many of these infillings being now seen standing out in 

 relief on the surface of the decomposed basalt. The silicated 

 waters gradually filled up the basins, and in process of time the 

 trees which had already begun to decay in the loose sands in which 

 they were lying, became silicified in the form in which they are 

 now found. 



Various writers have asserted that there were thermal springs 

 during Lower Miocene times, but there is no evidence for this. The 

 remains of plugged fissures are seen sticking up in Lower Miocene 

 deposits, but the fact that no alteration of those beds is discernible, 

 while in every instance where they occur in the Oligocene the sands 

 are consolidated round them, to the writer's mind effectually disposes 

 of this contention. Dr. Blanckenhorn ^ makes the age of the basalt 

 on the east side of the Nile Valley to be Lower Miocene, while he 

 assigns an Oligocene age to most of the silicified wood. The 

 evidence is all the other way, and the onus of proof rests with him. 

 As has been shown in a recent paper by the writer,- this basalt 

 underlies in a discordant manner beds of Lower Miocene age, and it 

 is therefore plain that the age of this lava is older than Lower 



1 Loc. cit. 



3 Geol. Mag., 1904, pp. 603-608. 



