46 Prof. J. D. Dana on some Results of 



No other cause of the gradual subsidence than that here cited 

 is appealed to. 



Now the whole of this contraction took place, if any occurred, 

 in the underlying Archcean rocks (Azoic, or Lauren tian and Hu- 

 ronian) ; for in obtaining by measurement this thickness (40,000 

 feet) the contracted rocks were measured. 



The 40,000 feet of subsidence required was therefore wholly 

 independent of contraction in the stratified sediments. But 

 these underlying Archaean rocks were probably crystallized 

 before the Palaeozoic era began ; for in New York and New 

 Jersey they are in this condition, and they underlie the Silurian 

 rocks unconformably ; and the New-Jersey Archaean or High- 

 land region is but a northern part of that of Pennsylvania and 

 Virginia. They would consequently have expanded with the 

 heat instead of contracting. Even if not crystallized, they 

 would have been well compacted under the enormous weight of 

 40,000 feet of strata ; and no experiments on rocks that I have 

 met with authorize the assumption that the ordinary law of ex- 

 pansion from heat would have been set aside. 



For further argument on this point I refer to the subsidence 

 in the Connecticut valley during the era of the Connecticut 

 river sandstone (supposed to be Triassico- Jurassic) . The thick- 

 ness of rock produced in the era was probably about 4000 feet ; 

 and this is the extent, therefore, of the registered subsidence. 

 The sandstone strata, as is apparent in many places, rest on 

 the upturned metamorphic rocks (gneiss, mica-schist, &c.) of 

 Palaeozoic or earlier age. As shown in the preceding para- 

 graph, the contraction, under Professor LeConte's principle, 

 must have been confined to the underlying rocks ; and since 

 these are crystalline metamorphic schists, and the depth of sand- 

 stone was not sufficient to raise much the temperature within 

 them (the rocks are in general little compacted and often feebly 

 solidified), the heat ascending from below as accumulation went 

 on above would have produced expansion instead of contraction. 



Without further reference to facts, it is, I think, clear that 

 the subsidence required could not be obtained by the method 

 appealed to by Professor LeConte. Whatever cause, in either 

 of the above cases, occasioned the subsidence, it must have been 

 one that could do its work in spite of opposition on the part of 

 the heat in the rocks themselves or those below. 



Another cause of local subsidence is local cooling beneath, ac- 

 companying the increasing accumulation of sediments. But 

 this idea is too obviously absurd to require remark. 



In the present state of science, then, no adequate cause of 

 subsidence has been suggested apart from the old one of lateral 

 pressure in the contracting material of the globe. 



