36 Prof. J. W. Judcl. Second Report on Specimens 



way between Tantah and Cairo. A sketch section, given by Sir 

 Samuel Baker, shows that in his view they form part of the original 

 floor on which the Delta deposits were laid down. 



In considering this suggestion of Sir Samuel Baker, however, it 

 should be borne in mind that the alluvial mud of the Nile alternates 

 with considerable masses of blown sand, and that the turtle-backs 

 may be the surfaces of great lenticular patches of such blown sand. 

 Communications which have been made to me by Colonel J. C. Ross, 

 R.E., C.M.G., formerly Inspector- General of Irrigation, seem to point 

 to the same conclusions as those arrived at by the late Sir Samuel 

 Baker. Colonel Ross states that the turtle- backs sometimes show a 

 distinctly shingly character like that of the materials derived from 

 the borings. 



Bearing in mind the extreme importance of determining the 

 geological age of the deposits underlying the delta-deposits, I have 

 made the greatest efforts to obtain contemporary fossils from their 

 materials. The clays have been most carefully washed and picked 

 over, and the sands sifted and searched ; but although derived fossils 

 are in some cases very abundant, not a single organism has been 

 found which lived when the beds were deposited, and would serve to 

 throw light on the geological period to which they must be assigned. 

 Even the fine clays between 151 and 153 feet, which, it was hoped,, 

 might have permitted of the preservation of some organic forms, 

 have proved to be hopelessly barren of fossil remains. 



As it was of considerable interest to determine the source of the 

 various pebbles making up the shingle deposit, which we may con- 

 veniently speak of as the " Sub-delta formation," I placed myself in 

 communication with Dr. Karl von Zittel, F.M.G.S., of Munich, who 

 possesses such a unique knowledge of the rocks and fossils of North 

 Eastern Africa. In his obliging communication, he has indicated the 

 probable source of the pebbles which I forwarded to him, and writes 

 as follows : — 



" The quartz and chalcedony pebbles from depths of 120, 160, 245, 

 and 270 feet are almost absolutely pure examples of those rocks. 

 The sandstones (for example, those from the depth of 120 feet) 

 rather recall, in their general appearance, the Tertiary Sandstone of 

 Gebel Achmar, near Cairo, than the older (Cretaceous) Nubian 

 Sandstone of Upper Egypt. The quartz and chalcedony pebbles, 

 before referred to, might also be derived from the Gebel Achm.ar 

 Sandstone. The absence of limestone pebbles is striking ; it would 

 appear that only the harder rocks have been preserved in the gravels 

 of the Delta, the softer ones having been possibly worn away." 



With respect to the igneous rocks found as pebbles in these shingle 

 beds, Dr. von Zittel suggests, from their macroscopic appearance, that 

 they may be derived " from the side valleys of the Arabian Desert." 



