on the Organic Composition of Chalk and Chalk Marl. 313 



Infusoria conglomerate of Oran, to vvhicli I have already ad- 

 verted. It is })robably what formed the Tripel of the earlier 

 periods of Italy. When M. Ilozet speaks (at p. 28-30) of 

 the great extent of the tertiary tract near Algiers as similar 

 in its relations to those of Oran, I cannot agree with him. On 

 the contrary, forming my judgment by the organic remains, 

 I consider the desert tract near Algiers as really composed of 

 a tertiary formation, which reposes on chalk. This opinion 

 is founded on my observation, that the tract in Libya, extend- 

 ing from Alexandria to Siwa, is composed of tertiary beds, 

 while from Cairo to Geza the chalk formation occurs, which 

 terminates at the granite of Syene, but is far spread into the 

 Desert. The valley of Siwa appears to form the northern 

 boundary of the chalk in Eastern Libya. 



In the South of Italy, at Caltasinetta and its neighbourhood, 

 the relations had been correctly seized by our late friend 

 Freilerick Hoffmann, from whose diary I have been favoured 

 with an extract by M. von Dechen. He represents the series 

 of strata which occupy the greater part of Sicily as composed 

 of limestones, sandstones, clays, and marls; the lower mem- 

 bers being probably referable to the Jura formation, suc- 

 ceeded by such as clearly belong to the chalk, and many beds 

 of which perfectly resemble the hard chalk of the north-west 

 of Germany (Teutoburger Wald). Among the marls are 

 white chalk-like thinly laminated masses, analogous to Tri- 

 pel, designated by Hoffmann as liohite chalk marl^ and which 

 especially occur in the southern part of the island. The beds 

 of the chalk formation usually dip 20° to 30°, while the strike 

 is nearly constant, from ] 5° to 45° S. of E. and N. of W., 

 parallel to the south coast. The tertiary beds which succeed 

 the chalk are composed of loose sand, friable sandstone, tes- 

 taceous breccias, clays and limestones. They cover the chalk 

 unconformably, resting on the truncated edges of the latter. 

 The chalk beds are upon the whole poor in organic remains, 

 and these are seldom distinct ; there occur Hippurites, Num- 

 mulites, Lenticulites, and in a few places indistinct Ammo- 

 nites and Belemnites, while the tertiary beds are quite lilled 

 with innumerable MoUusks, of which nine-tenths are still li- 

 ving in the Mediterranean. This distinction is so striking that 

 it scarcely required the difference of relative position in order 

 to draw a correct line between the two formations. Even had 

 so circumspect a geologist as Frederick Hofilnann not cor- 

 rectly seized and pronounced with decision on these local 

 relations, the numerous microscopic siliceous Infusoria with 

 calcaieous Polythalamia which I have found in the chalk 

 marl would have led to the same conclusion. 



