178 Revieics — Prof. Dr. Karl ZitteVs Geology, etc. 



whilst the interior portion is completely unaltered ; also the Nura- 

 mulites and other fossils are often silicified on their upper surface, 

 whilst the under side retains its calcareous constitution. These 

 phenomena indicate a process of slow superficial silicification, which 

 may he explained hy minute portions of silica dissolved in dew, 

 which f>:radually take the place of the carbonate of lime." 



III. Upper Eocene Series. — These were only seen in the extreme 

 westerly portion of the desert traversed hy Eohlfs' expedition, 

 and consist of limestone beds of about 10 m. in thickness, nearly 

 entirely composed of organic remains. The character of the Mol- 

 lusca indicates that these beds belong to the horizon of the Barton 

 Clay. Upper Eocene or Oligocene strata of a probably somewhat 

 newer date were discovered by Prof. Schweinfurth in a small island 

 in the Lake Birket-el-Qurun ; the vertebrate remains from these 

 beds have been already described by Dr. Dames. 



B. Miocene deposits are clearest shown on the northern margin of 

 the Oasis of Ammon, near the small town of Siuah. where an escarp- 

 ment 130 m. in height, of limestone and calcareous clays, with 

 abundant fossils, is displayed. This Miocene limestone near Siuah is 

 the last marine deposit in the Libyan desert, though it may probably 

 continue towards the north over the Cyrenaish plateau. In some 

 localities in the sandy desert between Eegenfeld and Siuah, beds of 

 freshwater limestone overlaid by quartz and sandstones, from 10 

 to 12 m. in thickness, are exposed ; the shells in them are insuf- 

 ficient to determine their distinctive horizon, but from their intimate 

 connection with the Lower Miocene marine beds, they may probably 

 belong to the Middle Tertiary period. Petrographically the sand- 

 stones resemble those which near Cairo contain the well-known 

 silicified trees. These trees are naked trunks, without branches or 

 roots, and destitute of bark ; they are strewn in great profusion 

 irregularly over each other in a horizontal direction. The wood has 

 been completely silicified, and the fibrous structure is so beautifully 

 preserved that it can be distinguished by the naked eye. Schenk 

 has recognized no fewer than nine species belonging to eight genera, 

 amongst which is a palm, a conifer, and seven dicotyledonous forms. 

 The age and the origin of these forests of silicified trees are both 

 uncertain. Considering their gigantic dimensions — some reaching 

 from 20 to 30 m. in length, and 3-5 m. in circumference, — and their 

 condition, it is impossible to suppose that they can have been trans- 

 ported any distance by water ; and it is probable that they grew near 

 where they are now found. The circumstance that these trees belong 

 to extinct species, and mostly to genera, which, if not extinct, do not 

 now exist in Egypt, points rather to their Tertiary than to their 

 Quaternary age. 



3. — Qmdernary and Becent Formations. — In the Libyan desert 

 these are comparatively rare, but it is probable that during the 

 Quaternary period the now southern desert regions were exposed 

 to the influence of an abundant freshwater supply, which excavated 

 the valleys and formed the steep terrace-margins and the island-like 

 hills. There is no doubt that the Nile at an earlier period was a 



