Geological Results of the Earth's Contraction. 89 



It should be remarked, that in the following summary the 



source 



causes alluded to are not 

 effects enumerated, though a legitimate and sufficient source. 

 The causes have acted conjointly with the wide-spread agency 

 of water, yet they may have been less dependent on the latter for 

 many results, than has often been urged. We mention no authori- 

 ties for any of the conclusions stated, as they are already given, as 

 far as known to the author, in the previous articles alluded to,* 



General Review of the Consequences of the Earths Cooling. 



perfect 



surface 



a. The change inconceivably slow, and hence the rock formed hav- 

 ing a coarsely crystalline texture :— the subsequent progress of solidifi- 

 cation beneath the crust still more gradual, and therefore producing at 

 all periods of the globe a coarsely crystalline texture : — the whole the 

 result of a single immeasurably prolonged operation.! 



&. Hence, probably, a general uniformity in the crystalline structure, 

 sufficient to give the crust apparently two directions of easiest fracture, 

 whose mean courses are N. W. b. W. and N.E. b. N. ; yet varying much, 

 being probably dependent to a great degree on the early direction of 

 isothermal and isodynamic lines, (this Journal, iii, 392.) 



c. In the progress of this cooling, commencing with its first begin- 

 ning, the surface necessarily presenting large circular or elliptical areas 

 that continued open as centres of fluidity and eruptive action,^ (ii,345; 

 in, 395.) Subsequently, a gradual reduction in size of these centres of 

 igneous action and their frequent extinction. 



We add here a reference to the valuable memoirs on slaty cleavage, by W. 

 ^narpe, Esq., in the Quart. Jour. Geol. Soc, No. 7, p. 309, and No. 9, pp. 74-105. 

 kee also this Journal, last volume, p. 430, and p. 110. in this number. 



See also on the effects of cooling, De la Beche's Report on Cornwall, Devon 

 and W. Somerset, 8vo, London, 1839, p. 33, and elsewhere. 



t Long sustained heat of a requisite and scarcely varying temperature, is the 

 essential circumstance demanded for the distinct crystallization of most minerals 

 from fusion. It is well known that lava streams after becoming inertia ted over, 

 ^e of len years in cooling. Yet they pass to the cold state too rapidly or irregu- 

 larly, f or a coarse crystallization of all the several ingredients of the rock, and thug 

 illustrate the absolute necessity of the condition stated. We have observed else- 

 vv «ere, that a granite-like structure is seldom produced about a volcanic vent ex- 

 cept m its central mass of lavas where they finally cool, shut out from the air by 

 tnick beds of non-conducting rock, (ii, 349.) . ._ 



we remark farther, that a long-continued uniform temperature, of some specific 



It is a 



structure, 



ae gree, is a condition of the greatest importance in chemical combination, 

 condition which the Author of nature has established in the animal s 

 wnere the most complex compositions take place. And when the requisite degree 

 01 beat in specific cases is ascertained, and the means of sustaining an unvarying 

 " „. i j i:„» .i.„. „„ m «> ».homi('al comoo.«itions will 



fmperature are .. ..„..„, „„ ...^ rw . „ - - . 



,. e m L a . de to take place directly, which now require indirect processes. I lie reason 

 ,V n,s ,s obvious, if we consider that with difference of temperature ifl connected 

 umerence of size, and difference of attracting power both cohesive and chemical. 



* Veil illustrated on the surface of the moon, as also are many of the points her* 

 mentioned, (ii, 335.) gee Beer and Madler's charts. 



s *cqND Series, Vol. IV, No. lO.-July, 1847. 1 



