I GEOLOGICAL MEMOIRS. 



little changed, except that at their exterior they appear to melt into 

 the surrounding mass. The rock has a remarkable resemblance to 

 those sandstones which have been long exposed to a very great heat in 

 a blast-furnace or glass-house, so that even the pure quartz and sub- 

 stances related to it have undergone a slight degree of fusion. 



" This fritted and half-molten sandstone covers a large area in the 

 desert, and entire hills are composed of it. Moreover, we meet with 

 this appearance not only in the diluvial sandstone district of Lower 

 Egypt, but likewise in the tertiary sandstones of Upper Egypt, in the 

 red sandstone of JMount Sinai, in the tertiary variegated red sandstone 

 of Nubia, and, with the exception of Lower Egypt, frequently in the 

 vicinity of outbursts of unstratified rocks, such as granite, porphyry, 

 trachyte, &c. It is this last circumstance especially which, on viewing 

 this remarkable appearance, suggests the idea of volcanic action ; for 

 we conclude, and to a certain extent must necessarily conclude, that 

 heat was the agent in the formation of the above-named crystalline 

 rocks. Nevertheless we come to places where one must have a strong 

 prepossession in favour of igneous action to be able to discover any 

 trace of volcanic agency. When we meet with exactly the same ap- 

 pearances in the widely extended plains of the desert, far distant from 

 the above-named unstratified rocks, without a trace of any elevation, 

 any fissure, or any outburst, we naturally ask, where can the focus of 

 volcanic action have been ? 



" It seems then that in this case we have to do with two powers, 

 which although essentially different, have in different ways produced 

 the same effects, — have given rise to the same appearances. The one 

 of these, volcanic action, caused by the vicinity of crystalline rocks, 

 in connection with their eruption ; the other, a separation of the sili- 

 ceous matter in the sandstone, and an after-precipitation of it in cer- 

 tain places, causing the particles of the sandstone to assume a more or 

 less homogeneous structure, — an action similar to that which gives rise 

 to siliceous concretions. There is however this difference in this last 

 process, that the concretions are constituted of the precipitated ma- 

 terial only, and form a simple mineralogical body ; whereas the con- 

 cretions in the case in question present the appearance of the particles 

 of the precipitated matter, not combining with each other onl}^, but 

 mixing up with the unaltered and altered constituents of the rock in 

 which the process has taken place, forming a new body, a newly com- 

 pounded stone, which becomes more homogeneous the greater the 

 action of the precipitated material, both in amount and in effect. 



*' Which of these two ways nature chose in each particular case I 

 will not venture to decide, and I therefore undertake only, as often 

 as this appearance presents itself, to describe the circumstances of 

 each locality in detail, leaving to those who are not contented with 

 knowing the facts only, to deduce from them such hypothesis or 

 theory, or by whatever name they choose to call it, as they may feel 

 inclined. 



*' One of the most remarkable places where this vitrified sandstone 

 may be seen is the Dschebel Achmar (the red mountain) north of the 

 Mokattam, and separated from it by a defile. This hill may be seen 



