26 INDUCTION. 



marks of blood, the footsteps of the supposed murderers, and 

 so on, proceeding throughout on uniformities ascertained by a 

 perfect induction without any mixture of hypothesis ; so if we 

 find, on and beneath the surface of our planet, masses exactly 

 similar to deposits from water, or to results of the cooling of 

 matter melted by fire, we may justly conclude that such has 

 been their origin ; and if the effects, though similar in kind, 

 are on a far larger scale than any which are now produced, we 

 may rationally, and without hypothesis, conclude either that 

 the causes existed formerly with greater intensity, or that they 

 have operated during an enormous length of time. Further 

 than this no geologist of authority has, since the rise of the 

 present enlightened school of geological speculation, attempted 

 to go. 



In many geological inquiries it doubtless happens that 

 though the laws to which the phenomena are ascribed are 

 known laws, and the agents known agents, those agents are 

 not known to have been present in the particular case. In 

 the speculation respecting the igneous origin of trap or gra- 

 nite, the fact does not admit of direct proof, that those sub- 

 stances have been actually subjected to intense heat. But 

 the same thing might be said of all judicial inquiries which 

 proceed on circumstantial evidence. We can conclude that a 

 man was murdered, though it is not proved by the testimony 

 of eye-witnesses that some person who had the intention of 

 murdering him was present on the spot. It is enough, for 

 most purposes, if no other known cause could have generated 

 the effects shown to have been produced. 



The celebrated speculation of Laplace concerning the 

 origin of the earth and planets, participates essentially in the 

 inductive character of modern geological theory. The spe- 

 culation is, that the atmosphere of the sun originally ex- 

 tended to the present limits of the solar system ; from which, 

 by the process of cooling, it has contracted to its present 

 dimensions; and since, by the general principles of mecha- 

 nics, the rotation of the sun and of its accompanying atmo- 

 sphere must increase in rapidity as its volume diminishes, 



