588 



NATURE 



'jDct. 2 1, 1880 



formation. Concerning the age of llicse sandstone rocks 

 a considerable amount of controversy lias taken place in 

 recent years, and they have been referred by 'different 

 authors to the Triassic, the Jurassic, and the Neocomian 

 systems. The fossils found by Overvveg and others, how- 

 ever, seem to leave no room for doubt that the real age of 

 the Nubian sandstone is the Cenomanian, or lower 

 portion of the Upper Cretaceous. 



Lying upon these sandstones are found great deposits 

 over 600 feet in thickness, consisting of dark green and 

 grey, finely-laminated marls in their lower, and of white, 

 earthy limestones in their upper part. These rocks con- 

 tain many characteristic Upper Cretaceous fossils, such as 

 Ananchyies ovatits, Ventriculites, and Riidistcs. These 

 Upper Cretaceous rocks have been found not only 

 forming the whole southern margin of the Desert, but also 

 rising above the sandy wastes in the hilly masses which 

 form the oases. 



The deposits which underlie the greater part of the 

 Sahara appear to be of Tertiary age and referable 

 to the Nummulitic and Miocene periods. There would 

 seem to be no sharp line of demarcation between the 

 Cretaceous and the Tertiary deposits in this area, and in 

 this, as in many other particulars, which are pointed out by 

 Prof Zittel, the North African formations of these periods 

 remind us of those of [the Rocky-Mountain regions of 

 North America. 



The older Tertiary deposits of Northern Africa are 

 divided by Dr. Zittel into two members, which he desig- 

 nates the " Lybysche Stufe" and the " Mokattam Stufe."" 

 In the lowest of these (the Libysche Stufe) a widely- 

 spread and very characteristic fossil is the Belemnite-like 

 Graphulatia descytorui}!, Zitt. ; many nummulites and 

 other well-marked Eocene fossils also occur. 



There appears to be still some doubt as to whether the 

 "Mokattam Stufe" of Dr. Zittel should be Classed as 

 Eocene or Uligocene. 



In the northern part of the area various freshwater 

 and marine deposits are found which are now referred to 

 the Miocene. No less than si.\ty-eight forms of marine 

 mollusca have been determined by Dr. Theodor Fuchs 

 as occurring in the-e beds, and he is led to regard them 

 as indicating a horizon not far removed from that of the 

 Leitha-kalke of the Vienna basin. 



The several formations described succeed one another 

 from south to north, this being the direction of the dip 

 of the beds; their relations to one another are well 

 illustrated in the map and sections which accompany the 

 work. 



In the midst of the Beharieh oasis a mass of igneous 

 rock is found rising through the midst of the Upper 

 Cretaceous limestones. This rock has been studied by 

 Prof. Zirkel of Leipzic, who pronounces it to be an ordi- 

 nary plagioclase basalt, very similar in character to that 

 of the Giant's Causeway in Ireland. 



Over the whole of these formations the great mass of 

 sands of the Desert is spread, and rises in places into 

 hills several hundreds of feet in height. 



In reading this address we cannot but feel that Dr. 

 Zirkel has made admirable use of the collections which 

 Dr. Schweinfurth and others have placed in the museum 

 at Munich ; and that by their careful study he has been 

 enabled to clear up many of the difficulties which every 

 one must have felt who has endeavoured to understand 

 the geological structure of the great African continent. 



PHYSICS WITHOUT APPARATUS^ 



VIII. 



TN the preceding articles of this series we have shown 



how m every department of physics a large number 



ot mstructive experiments may be performed without the 



aid of any more formal apparatus than the usual domestic 



^ Continued from p. 538. 



appliances of an ordinary household. There remain to 

 be described a few miscellaneous experiments before 

 concluding tlic subject. 



Many years ago Prof. Faraday observed that if two 

 pieces of ice are pressed against one another they freeze 



firmly together at the point of contact, even though they 

 may themselves be thawing at the surface. To this 



