90 , Geological Results of the Earth? s Contraction. 



d. A boiling movement or circulation (up at centre and down around 

 the sides) in the vast circular areas of igneous action, owing to escap- 

 ing vapors, and dependent mainly on the temperature being greatest 

 below at centre and least at the surface and laterally.* As this circu- 

 latory or cyclosis movement occurs in material whose mineral ingredi- 

 ents or products differ in the temperature of solidification or of forma- 

 tion, it determines to some extent the distribution of these mineral con- 

 stituents, and of the rocks which are formed. In later periods, this 

 cause producing a feldspathic centre to volcanic mountains having basal- 

 tic sides, (ii, 343.) 



e. As refrigeration went on, the centres of eruption becoming mostly 

 extinct over large areas, and remaining still active over other areas of 

 as great or greater extent: — for cooling, wherever commenced, would 

 extend somewhat radiately from the centre where begun, (yet with some 

 relation to the structural lines,) and so gradually enlarge the solidifying 

 area and encroach upon the more igneous portions. 



II. Contraction, as a consequence of solidification, attended by 

 a diminution of the earth's oblateness. 



a. Rate of contraction in different parts unequal, according to the 

 progress of refrigeration ; and after the formation of a crust, greater 

 beneath the crust than in the crust itself, (iii, 96, 181.) 



I. Contraction beneath the crust causing a subsidence of the surface. 



c. Subsidence greatest where the crust was thinnest or most yielding, 

 and least in those parts which were thickest from having been first 

 stiffened by cooling; — the large areas that continued to abound in igne- 

 ous action therefore becoming in process of time more depressed than 



i 



i 



those areas that were early free (or mostly so) from such action, (ii 

 352; iii, 181.) 



^ d. Subsidence of the surface progressive; or, if the arched crust re- 

 sisted subsidence, a cessation, until the tension was such as to cause 

 fractures, and then a more or less abrupt subsiding, (iii, 96.) 



e. Frequent changes and oscillations in the water level, either grad- 

 ual or abrupt, arising from the unequal progress of subsidence in dif- 

 ferent parts, and also in early periods from extensive igneous action 

 (iii, 95, 181.) 5 



III. Fissures and displacements of the crust, owing to the 

 contraction below it drawing it down into a smaller and smaller 

 arc ; also, from a change in the earth's oblateness. 



a. Fissures influenced in direction by the structure of the earth's 

 crust,— because of the existence of such a structure, and also because 



* The boiling action in Kilauea, Hawaii, appears in general character, closely 

 like that of boiling water. In the great lake, 1500 feet in diameter, there is an ac- 

 tive play of jeis over the surface precisely as in a boiling fluid, with no sounds 

 ordinarily but the grum murmur of ebullition. A constant flow is seen in the 

 liquid, (well shown in the jets that move with the current,) from the hottest part, 

 near the northeast side, towards the southwest part of the lake ; and this flow is 

 so remarkable that it was formerly accounted for by supposing that a submarir 

 stream of fire here came to the surface, and disappeared a^ain after being for a 

 short distance visible. 



















