THE REVOLUTIONS OF THE CRUST OF THE EARTH. 289 



the dissolution of the plutoniau rocks, -which were deposited under the 

 form of unstratified sediment iu proportion as the water cooled. 



After the precipitation or* the water, calm did not reign upon the 

 surface of the earth ; violent reactions evinced the powerful but irregu- 

 lar struggle of the central forces. The origin of these forces is found 

 iu the constant loss of the heat of the globe. According to Eeich and 

 Bailey the density of the earth must be between 5.44 and 5.G7. It is, 

 then, greater toward the center than at the surface. This is evident 

 from the mean density of the sedimentary and eruptive rocks of the 

 crust. 



We ought not to be surprised at the great density of the central 

 strata; on the contrary, if we take into account the compressibility of 

 liquids, and the enormous pressure the subterranean mass supports 

 we will be astonished that it is not greater, and seek, with Lesley, a force 

 sufficiently powerful to counterbalance this immense pressure. We 

 know that the heat alone maintains bodies in a certain state of fluidity ; 

 but as there is a constant loss of heat, equilibrium must cease and the 

 entire mass shrink in proportion to the loss of caloric. The terrestrial 

 crust which passes from the soft to the liquid state contracts more than 

 the total volume of the ignited mass; a cracking of the crust ensues 

 and an ejection of melted and incandescent matter. 



The sedimentary crust does not contract in any way. It sinks at 

 certain points and rises at others, following the contraction of the 

 solid crust it covers. Hence the heaping up in places, the rents and 

 irregularities of a surface once perfectly united and level. All these 

 inequalities are so many new agents for the circulation of the water at 

 the surface of the globe, which by disaggregation and denudation fur- 

 nishes material for the filling up of oceanic depressions. 



The appearance of organisms in their most simple forms soon followed 

 the formation of the sedimentary strata. This was a sign that the 

 chaotic state which necessarily prevailed during the period of the pre- 

 cipitations of the first waters was approaching its end. We know little 

 of the structure of these first orgauisms. They were only the precursors 

 of others more perfect, which in their turn became the antecedents of the 

 existence of beings of a still higher order. 



There have been various efforts made to estimate the age of the earth, 

 at least since the commencement of the sedimentary formations. Hum- 

 boldt has said that geologists by their hypotheses and their conclusions 

 penetrate into spaces aud times which are far beyond the limits of our 

 observation. But it should not be forgotten that if in this way they 

 can arrive at a certain probability or relative certainty, it is rarely ac- 

 corded them to settle their conclusions upon indisputable and necessary 

 data, and therefore we cannot place entire faith iu the estimations of the 

 age of the terrestrial crust. Mr. Bischof has endeavored to show by 

 the cooling of basalt and of lava that the earth has taken nine millions 

 ,of years to cool down to the point it is at present, provided it be ac- 

 s 19 



