544 HISTORY OF GEOLOGY. 



which may enable us in some measnre to judge to what point this 

 division of our subject is tending. 



We may remark, in this as in former cases, that since we have here 

 to consider the formation and progress of a science, we must treat as 

 unimportant preludes to its history, the detached and casual observa- 

 tions of the effects of causes of change which we find in older writers. 

 It is only when we come to systematic collections of information, such 

 as may afford the means of drawing general conclusions ; or to rigo- 

 rous deductions from known laws of nature ; that we can recognize 

 the separate existence of geological dynamics, as a path of scientific 

 research. 



The following may perhaps suffice, for the present, as a sketch of 

 the subjects of which this science treats : the aqueous causes of 

 change, or those in which water adds to, takes from, or transfers, the 

 materials of the land : the igneous causes ; volcanoes, and, closely 

 connected with them, earthquakes, and the forces by which they are 

 produced ; the calculations which determine, on physical principles, 

 the effects of assumed mechanical causes acting upon large portions of 

 the crust of the earth ; the effect of the forces, whatever they be, 

 which produce the crystalline texture of rocks, their fissile structure, 

 and the separation of materials, of which we see the results in metal- 

 liferous veins. Again, the estimation of the results of changes of 

 temperature in the earth, whether operating by pressure, expansion, or 

 in any other way ; the effects of assumed changes in the superficial 

 condition, extent, and elevation, of terrestrial continents upon the cli- 

 mates of the earth ; the effect of assumed cosmical changes upon the 

 temperature of this planet; and researches of the same nature as 

 these. 



These researches are concerned with the causes of change in the 

 inorganic world ; but the subject requires no less that we should in- 

 vestigate the causes which may modify the forms and conditions of 

 organic things ; and in the large sense in which we have to use the 

 phrase, we may include researches on such subjects also as parts of 

 Geological Dynamics ; although, in truth, this department of physi- 

 ology has been cultivated, as it well deserves to be, independently of 

 its bearing upon geological theories. The great problem which offers 

 itself here, in reference to Geology, is, to examine the value of any 

 hypotheses by which it may be attempted to explain the succession of 

 different races of animals and plants in different strata ; and though 

 it may be difficult, in this inquiry, to arrive at any positive result, we 



