26 GEOLOGICAL MEMOIRS. 



the outskirts of the vast plains of the Pampas. As little can it ex- 

 tend westwards towards the Pacific Ocean. A considerable elevation 

 on the mountains must be attained before it is met with. What then 

 can induce the chalk to run along, only in the direction of the high 

 mountain ridge of the volcanos, and only in a narrow band, and never 

 to descend into the plains !! In the whole of Brazil, in the wide re- 

 gions of La Plata, Paraguay, Bolivia, the chalk is never again seen, 

 and indeed does not exist. Is it not like a band of chalk which has 

 been formed along the volcanic fissure of the Andes before the moun- 

 tains were elevated, perhaps because the slightly concealed fissure had 

 produced the conditions of life and existence for the cretaceous mol- 

 lusca on a more extensive and easily attainable scale ? 



Darwin has followed the cretaceous strata to the extreme point of 

 the continent. He saw cretaceous shells in abundance on the top of 

 Mount Tarn 2000 feet high, near Port Famine in the Straits of Ma- 

 gellan, and in 53 degrees of south latitude, and consequently three 

 degrees of latitude higher than on the Missouri. Ancycloceras sim- 

 plex, d'Orb. and Hamites elatior. Sow. leave no doubt of the chalk. 

 The Hamite is even, says Prof. Edward Forbes, one of the largest 

 ever seen, fully 2\ inches in its largest diameter. Darwin's discovery 

 probably determines the most southern hmit of the cretaceous forma- 

 tions ; and hence polar influences may have here also opposed its 

 further extension towards the Pole. [J. N.] 



Guide to Geognosy {Leitfaden und Vademecum der Geognosie), 

 by B. CoTTA. Dresden u. Leipzig, 1847. 8vo. pp. 292. 



This work forms the third edition of the author's * Elements of 

 Geognosy and Geology ' (Grundriss der Geognosie und Geologic), but 

 very considerably modified in its arrangement and mode of treatment. 

 It commences with a short sketch of Geognosy, which the Germans 

 limit to the * Knowledge of the internal structure of the solid mass of 

 the earth' ; whilst Geology is the ' Doctrine of the origin of this struc- 

 ture and the investigation of the causes of the present condition of the 

 earth generally.' The first section treats of ' External Geognosy,' or 

 the general structure of the earth, its density, magnetism, tempera- 

 ture, form, and the influence of water, volcanic heat, and organic life 

 on its surface. The second contains ' Internal Geognosy,' or the in- 

 terior structure of the solid crust of the globe ; the third, the ' Doctrine 

 of Rocks {Gesteinslehre),'' their mineral composition, texture, form 

 and divisional structure. Then follows an account of the general 

 principles of petrifaction and stratification, and of the chief stratified, 

 slaty and massive rocks, with a notice of the nature and theory of 

 veins. To this succeeds an extensive Index or Alphabetical explana- 

 tion of the common designations of rocks and formations, with refer- 

 ences to the works — chiefly foreign and especially German — in which 

 they are more fully described. The whole concludes with three tables 

 showing the distribution of Organic remains, the Succession of rocks 

 in Germany, and a Comparison of the succession in that country with 

 some other regions of the earth. 



