389 



ported Cycadeae, then a lake or estuary in which freshwater strata 

 were deposited, then again land on which other Cycadeae and a fo- 

 rest of dicotyledonous trees flourished ; then a second submergence 

 under fresh water, in which new strata were formed ; and finally, a 

 return of the ocean in the South-east of England, when the green- 

 sand and chalk were superimposed upon the Wealden. The ap- 

 pearances in Portland alluded to by Dr. Fitton may be explained 

 either by the alternate rising or sinking of the same ground, or by 

 simply supposing one gradual and continuous subsidence in a region 

 where a large and turbid river entered the sea. The conversion of 

 certain tracts into land several feet high might be caused in a single 

 year by river-inundations, and there might be sufficient time for a 

 forest to grow upon these before the continued sinking down of the 

 land (assuming it to have been constant) had time to cause the tract 

 to be again submerged. I have before adverted to the petrified 

 forest described by Mr. Darwin, in Chili, where the trees have grown 

 on a bed of lava, and have then been covered by sand and sedi- 

 mentary and volcanic matter 2000 feet thick. These facts seem to 

 prove that the region of the Andes, instead of having been raised 

 up suddenly and at once, a few thousand years before our time, as 

 some have conjectured, has undergone, even since the commence- 

 ment of the tertiary period, vast movements of depression as well 

 as of elevation. 



Among the modern changes of the surface of the globe which 

 have been attributed to a depression of the earth's crust, I may men- 

 tion the great cavity in Western Asia spoken of by Humboldt in his 

 Asiatic Fragments. The supposed existence of a region of dry land 

 1 8,000 square leagues in area, surrounding the Caspian Sea, and be- 

 low the mean level of the ocean, naturally excited the most lively 

 curiosity. The fact was regarded for twenty years as established 

 by a series of barometrical measurements made in 1811 by Profes- 

 sors Engelhardt and Parrot. The difference of level which these 

 travellers assigned to the Caspian and Black Seas amounted to about 

 350 feet. But Professor Parrot, having revisited the tract in 1 829 and 

 1830, soon found reason to doubt the accuracy of his former conclu- 

 sions. He learnt that some Russian engineers had ascertained by 

 careful measurements that the Don, at the place called Katschalinsk, 

 where it is only sixty wersts distant from the Wolga, is 130 Paris 

 feet higher than the latter river, and as the Don flows with much 

 greater rapidity to the Black Sea than the Wolga does to the Cas- 

 pian, the difference of level between the two seas, if any, must be 

 considerably less than 130 feet. Parrot accordingly made a series 

 of levellings from the mouth of the Wolga to Zarytzin, 400 wersts 

 up its course, and from the mouth of the Don to the like distance ; 

 and these observations gave as a result that the mouth of the Don 

 was between three and four feet lower than that of the Wolga ! 

 So that, according to this measurement, if there is any difference 

 between the levels of the two seas, the Caspian is the highest ! 

 Baron Humboldt, who with other geographers had given full credit 

 to the former statement of Parrot, very naturally refused to admit 



