Mr J. D. Dana ow the Origin of Continents. 237 



the area ; and at the time of Assuring, there might be other 

 upheavals. It follows, hence, that — 



a. There would be prolonged intermissions in the subsi- 

 dence of given areas ; and this must have been the fact 

 throughout the history of the globe. 



6. There must have been oscillations in the land as com- 

 pared with a water level, the water at times rising gradually 

 over land that, during a previous period, had emerged ; and 

 the reverse. 



c. There might be in the same epoch, under such circum- 

 stances, an unequal retreat of the ocean from the coasts of 

 different continents, or a rise in one place and a retreat in 

 others ; for the changes by contraction are supposed to have 

 been everywhere in progress at the same time, and through- 

 out different in character and extent. 



d. Changes of level may in some cases have been gradual^ 

 and in other cases paroxysmal ; for the opening of large 

 fissures would often be of the latter character. 



8. In an elliptical area of contraction, there will be two 

 systems of fissures at right angles with one another, as fol- 

 lows from the calculations of William Hopkins, Esq.* But 

 if the area is bounded on one side by a region participating 

 but little in the contraction, the efi*ects would be most de- 

 cided on the borders of such a region ; and they would con- 

 sist in extensive fissures ranging along the area, and an 

 attending swelling of the surface, or else a rising of the 

 strata into folds by lateral pressure.t 



* Trans. Camb. Phil. Soc, vii., 22. 



t With regard to the folding of strata by lateral pressure, the theory was 

 first presented by Sir James llall, (Trans. Roy. Soc. Edinb., vii., 85,) and the 

 injection of granite, coupled with the elevation of the land, was suggested by 

 him as a probable source of the pressure in the instances he mentions. Scrope, 

 reasoning on this subject, says, in his work on volcanoes, published in 1825, 

 " There is reason to conclude that in most instances, the raised strata, par- 

 ticularly those which were only partially indurated, have been contorted and 

 bent into repeated foldings, so as to give the appearance of frequent alterna- 

 tions of different series of strata to what is in reality but the replication of 

 the same original series." — P. 201. De la Beche applies the theory to the 

 structure of the Alps, (Geol. Researches, 129,) the possibility of which applica- 

 tion was suggested by Sir James Hall. 



Authors have generally followed Sir James Hall in considering that besides 



