294 American Scientific Association. 



there is a general projection or hollowing of the under surface 

 corresponding; in a word, the general outline of the two surfaces 

 is similar." If the surface of the earth is raised by continents, a 

 corresponding thickness or elevation must be found inside, a swell- 

 ing inward of the crust ; and if the outer surface is depressed as 

 in ocean bottoms, there the inner surface is hollowed out, making 

 the middle of the bottom much thinner than the edges. The 

 speaker from the evidence adduced to prove these general ideas, 

 assumed that the centre of the earth was fluid, that the crust floats 

 upon its surface and is subject to the laws of floating bodies. 

 The laws and conditions under which this crust cooled and its 

 state when solidified were then scientifically explained at length, 

 as tending to confirm the generally accepted theories as to the 

 fluidity of the central mass. 



This theory, the speaker remarked, would satisfactorily ac- 

 count for the distribution of volcanoes, if not for the phenomena. 

 He admitted that volcanoes were the most difficult of explanation 

 of all the igneous phenomena in nature, and although gases and 

 vapours are probably one cause of the eruptions, yet he thought 

 few physical geologists would admit the local pressure of gas as 

 the only or even the chief cause. The great general cause, he 

 thought, might be the reaction of the crust upon the interior fluid, 

 and gave his reasons therefor. At any rate the disruption of 

 the crust should take place in the thinnest part as the bottom of 

 the sea, and the next place should be the next weakest part or 

 the margins of the sea, and these are exactly the places where the 

 volcanoes occur. Of 225 active volcanoes mentioned by Humboldt, 

 155 are situated upon islands in the ocean, and of the remaining 70 

 almost the whole are situated near the sea-shore, while but very 

 few are found in the interior of continents. This paper as a 

 whole was remarkably clear, logical and conclusive, and presented 

 many points worthy of study. 



GYPSUM AND MAGNESIAN ROCKS. 



Mr. T. Sterry Hunt, of Montreal, showed that besides those 

 gypsums formed by the alteration of beds of limestone, another class, 

 by far the more important, comprehends those gypsums which have 

 been deposited directly from water. Such may be produced during 

 the evaporation of sea-water ; but Mr. H. has recently shown that 

 sulphate of magnesia is decomposed by solution of bicarbonate 

 of lime, giving rise to gypsum, which is first deposited, and a 



