30 Mr J. D. Dana on the Volcanoes of the Moon. 



atmosphere or waters of the earth were not too warm for 

 tlie more rapid rate of cooling required for uncrystalline 

 rocks. Scoriae, basalt, trap, porphyry, syenite, granite, have 

 no velations to one epoch rather than another, beyond what 

 may tlepend on the circumstance just mentioned. Whenever, 

 therefore, in the history of the world, the variations in heat, 

 pressure, and rate of cooling, now possible, may have taken 

 place, similar rocks to those of the present day may have 

 been in progress : — and as far as the variations of former 

 times, in these respects, may now take place, former rocks 

 and minerals may still be in progress. In this statement it 

 is implied that the necessary elements are present in the 

 fused material. 



III. Origin of Continents. — The moon gives us hints on 

 another topic of great interest, relating to the distribution of 

 land find water on our globe. We have mentioned that there 

 is a large area covering nearly one-third of the hemisphere 

 facing the earth, which is mostly free froni volcanoes, while 

 on other parts the craters are plosely crowded together. Wei 

 may therefore reasonably infer, that over this naked portion 

 the surface first beoame solid, and has therefore cooled the 

 longest'and to the greatest depth. Consequently, the con- 

 traction from cooling, which was going on, would take place 

 most rapidly over the thinner and more yielding volcanic 

 portion ; and unless the ejections made up the difference, this 

 part would become somewhat depressed. A melted globe of 

 lead or iron in the same manner, when cooling unequally, 

 becomes depressed by contraction on the side which cools 

 last. Now on our own globe, the continents have, to a very 

 great extent, been long free from volcanic action. A glance 

 at a map of Asia and America will make this apparent. It 

 is usual to attribute this almost total absence of volcanoes 

 from the interior of the continents to the absence of the sea ; 

 but it is fatal to this popular hypothesis, that the same free- 

 dom from volcanoes existed in the Silurian period, when 

 tliese very continents were mostly under salt water, a fact 

 to which the wide-spnead Silurian rocks of America and Rus- 

 sia testify. Over the oceans, on the contrary, all the islands 



