MOVEMEXTS AXD DEFORM AT lOXS OF THE EARTH'S BODY. 575 



practical question, however, relates to its competency. Can the amount 

 of lava that has been extruded have had any veiy- appreciable effect 

 on the descent of the crust'? The great Deccan flow is credited \\ith 

 an area of 200.000 square miles, and a thickness of 4000 to 6000 feet. 

 Vast as this is for a lava-flow, it would form a layer only about 5 feet 

 thick when spread over the whole surface of the globe, and hence the 

 sinking to replace it would cause a lateral thrust, on any great circle, of 

 about 31 feet only. It requires a ven' generous estimate of the lavas 

 poured out between any two great mountain-making periods since the 

 beginning of the well-kno^n stratigraphic series to cause a horizontal 

 thrust of any appreciable part of that involved in mountain-making. 

 The case is different, however, if we go back to the Archean era. in 

 which the proportion of extrusive and intrusive rocks is ver}' high. 

 Ver}' notable distortion may then be assigned to the extravasation of 

 lavas. The outward movement of lava must also be credited with some 

 transfer of heat from lower to higher horizons, and this is probably 

 one of the agencies that ha^-e produced the relatiA'ely high imderground 

 temperatures in the outer part of the earth. 



If lavas are thrust into crevices of the crust they contribute to its 

 extension, but causes for the crevices and for the intrusion must be 

 found, and these are probably only expressions of one or another of 

 the more general agencies. 



Change in the rate of rotation. — As previously noted, the tide acts 

 as a brake on the rotation of the earth. The oblateness of the present 

 earth is accommodated to its present rate of rotation. It is assumed 

 that such accoromodation has always obtained, and that if the rotation 

 has changed, the form of the earth has changed also. Xow. the more 

 oblate the spheroid, the larger its surface shell and the less the total 

 force of graA^ty. Hence if the earth's rotation has diminished, its 

 crust must have shrunk, because the form of the spheroid has become 

 more compact, and the increase of gravity has increased its density. 

 There is at present a water-tide chiefly generated in the southern ocean, 

 and irregularly distributed to more northerly waters. This irregu- 

 larity interferes with its systematic action as a brake, and its average 

 effects are difficult of estimation. The water-tides of past ages are 

 stiH more uncertain, as they must have depended on the configuration 

 and continuity of the oceans. There are geological groimds for the 

 behef that the southern ocean was interrupted by land during portions 



