422 Revieics — MM. Bertrand 8^ Zurcher — 



Pacific Slope. — Beyond the great cutting the lignite system falls 

 below sea-level, but the limestone with Pecten subpleuronectes 

 represents the same fossiliferous horizon as on the Atlantic side. 

 The trachytic tuffs of the Pacific side are more recent than this, 

 i.e. than the base of the Lower Miocene, an opinion, as we have 

 already seen, opposed to the views of Hill, In speculating as to the 

 formation of the Isthmus generally, the authors especially dwell on 

 the slight consideration which should be attached to the difference 

 between the marine deposits on the Atlantic side and the brackish- 

 water deposits with lignite, leaves, etc., on the other. This they 

 think tends to minimize the importance of the hombement since the 

 Oligooene epoch. 



Latest Eruptions : Dividing Bidge. — With the formations noticed 

 above tlie sedimentar}'^ series of the Isthmus terminates, but the 

 eruptions have continued since the deposition of the Lower Miocene : 

 these last eruptions in the neighbourhood of the canal constitute the 

 summits of the actual crest. We have already seen that they are 

 described as andesites. It is only possible, say the authors, to fix 

 a lower limit to the age of these rocks ; they traverse in dykes the 

 Lower Miocene, to which their flows are superposed. " But what 

 one can say is that they have no connection with the recent volcanic 

 series known in Costa Rica, Nicaragua, and Guatemala, a series of 

 which the actual volcanoes are only the enfeebled prolongation. All 

 observers, and especially Mr. Hill, are very explicit on this point. 

 Now the earthquakes of the region are directly connected with 

 volcanic eruptions ; it is then an essential point to know that volcanic 

 activity has long since been extinct in the region of Panama." 



This account should not terminate without an allusion to the argiles 

 rouges superjicielles, which are the result of the disintegration of all 

 the above-mentioned formations, both igneous and sedimentary, under 

 atmospheric agencies well known in the tropics. This material occurs 

 on the surface and covers the several formations unconformably. It 

 functions apparently as a kind of drift and is very apt to slide, to 

 the great detriment of the canal cuttings. A description of the 

 Quaternary beds and alluvium follows. From the circumstance that 

 no Pliocene beds are found on the Atlantic coast the authors infer 

 that the sea was more distant at that period than at present. An 

 elaborate microscopic description of the rocks concludes this memoir. 



IL 



Mons. Bertrand alone is responsible for the second memoir, which 

 is illustrated by a map of the volcanoes of Central America after 

 Mons. Sapper (Zeitschr. deutsch. Gesellschaft, 1897), and also by 

 another map, showing the lines of folding and the lines of volcanoes 

 in Central America. In describing the four great volcanic ranges, 

 viz., those of Guatemala, San Salvador, Nicaragua, and Costa Rica, 

 the author observes that these are arranged in echelon, and he is 

 careful to indicate that the points of fracture of each of the volcanic 

 series correspond to the lines of exceptional mobility which are 

 marked by the existence of a lake or equivalent depression. The 



