28 PRELIMINARY INQUIRIES. 



is marked as tertiary in Map 3, and as seems more than probable of a p^reat continent in 

 the Atlantic, must have left a vacuity in the interior of the earth, and withdrawn a support 

 from the crust which stretched over it. That crust is both solid and elastic, but wc know, from 

 what takes place on the withdrawal of props in coal-mines, that the strata forming it are unable to 

 support themselves by cohesion ; and it stands to reason, that the crust of the earth, although 

 so much thicker than any strata existing above the beds of coal, being of correspondingly 

 greater extent, will be as little able to do so. The greater thickness will be compensated by the 

 greater extent without support. The supposed subsidence in the Paciiic Ocean extends over thousands 

 of miles, and an unsupported roof that woidd cohere for such an extent, must be made of other 

 materials than those of any minerals or rocks yet known. The elevation of the land in the northern 

 hemisphere must, therefore, have entailed a corresponding amount of subsidence elsewhere, and as 

 there is no " elsewhere " under water of corresponding dimensions but the Southern hemisj)here, it 

 must have been in the Southern Seas that the subsidence took place. The elevation and subsidence 

 being compensatory must have also been nearly simultaneous, not quite, indeed, for the elevation 

 being the cause of the depression must have been first in order of time. The elevation we know has 

 been subsequent to the commencement of the tertiary period. The depression therefore must have 

 been a little nearer to our own time. 



The sinking of the bottom of the Southern Ocean has probably been in operation long before 

 the present rising of the Javan band of volcanoes, and the latter belongs to a different chapter 

 in its history, and is, perhaps, more of the nature of an episode than an integral part of one 

 operation. The former may be a chapter nearly past, the latter the commencement of a new one. 



Applying this view to the occurrences since the eocene epoch, we have at least one great 

 result, the subsidence of the Southern hemisjphere, and the elevation of the Northern : perhaps 

 accomplished by alternate elevation and depression of portions of each. If the depression in the south 

 was, as I suppose, general over a considerable part of that hemisphere at one time, we may fix the 

 date of the subsidence of a large part of it. The geology of South Africa and the zoology of 

 Madagascar enable us to do this. 



Africa, south of the Sahara, has probably remained stationary since the secondary ej)och, 

 at least has not been submerged. " Judging from all the evidences as yet collected," says Sir 

 Roderick Murchison,* " the interior of South Africa has remained in that condition " (ter- 

 restrial and lacustrine only) " since the period of the secondary rocks of geologists." A nai'- 

 row belt of tertiary formations along the eastern coast, from the Cape to the Zambesi, and along 

 the south-west coast near Cape Negro, alone attests a trifling rise and extension of surface ; 

 Sir Roderick adds, " In truth, therefore, the inner portion of Southern Aii'ica is in this respect 

 geologically unique in the long conservation of ancient terrestrial conditions. This inference is 

 further supported by the concomitant absence throughout the larger portion of all this vast area, 

 i. e. south of the equator, of any of those volcanic rocks which are so often associated with oscillations 

 of the terra Jirma." 



To this continent Madagascar was at one time united, when it received the original elements of 

 its fauna and flora. These are of a type subsequent to the eocene date, and akin to luodcrn species. 

 Before the separation, therefore, the miocene epoch must have commenced. IIow was the separation 

 caused ? Most probably not by the sinking of the channel between them, but by the general 



' See his opeuing address totlie Geographical Society in 1864. 



