SOUTHERN MEXICO AND CENTRAL AMERICA 



705 



Meaning of Yucatan and Cayman Depressions 



The thin crust under the Yucatan and Cayman troughs, as previously 

 explained, might indicate immediately to the proponent of continental 

 drift, that the crust has been stretched and is in the process of being 

 fragmented or pulled apart. Even one not ready to accept large scale 

 drift of the continental masses may concede some pulling apart and 

 thinning. If stretching and thinning is admitted as a possibility, then the 

 thin gabbroic layer under the Mexican basin may also represent drifting 

 apart there, but at a somewhat earlier time because of the thicker layers 

 above of consolidated and unconsolidated sediments. Right off, however, 

 the seismic evidence does not suggest that we are dealing with a silicic 

 layer in the Cayman region — simply the gabbrioc layer is being thinned. 

 In continental drift, if it occurs, are we dealing with the movement of the 

 total crust over the mantle with a new gabbroic layer forming immedi- 

 ately in the breach, and progressively as it widens, or does the silicic 

 layer slide over the gabbroic? 



Postulated Wrench Faults 



With the discovery of the great fracture zones in the eastern Pacific 

 the attempt has been made to connect one of them, the Clarion, with the 

 Cayman trench across Central America. Hess and Maxwell's (1953) 

 postulate of Fig. 42.12 shows Honduras, Nicaragua, and El Salvador to 

 have moved eastward over 200 miles from a former position in what 

 is now the Pacific Ocean, along a break that would transect Central 

 America from southern British Honduras to the Pacific coast at about 

 the Chiapas-Guatemala border. Such a break would transect the Permian 

 fold belt and the metamorphic belt, for which there is no existing geologic 

 evidence. Also the Central American trench is a late Cenozoic feature 

 which, because it is continuous, precludes horizontal translation of 

 crustal blocks across it during this time. Since the Cayman trench is a 

 late Cenozoic feature, the movement postulated by Hess and Maxwell 

 has to be late Cenozoic, which is impossible across Central America. 

 The Clarion fracture zone takes off northwest of Acapulco and does not 

 line up with a projection of the Cayman trench. 



The zone of stratovolcanoes across southern Mexico might also be re- 

 garded as the line of horizontal movement, but no geologic relations 

 are known there to denote a wrench fault zone other than the aligned 

 vents. 



MAMMALIAN FOSSIL RECORD AND LAND CONNECTIONS 



The record of mammals, both existent and fossil, in North and South 

 America and in the isthmus itself is impressive, and speaks more posi- 

 tively of land connections than the physical, but still the two lines of 

 evidence lead to parallel and supporting conclusions. 



South America may have had southern and eastern connections during 

 the Mesozoic, but since late Cretaceous time at least, it has been isolated 

 from all the rest of the world, except for occasional connections with 

 North America (G. W. Simpson, personal communication). This lias 

 established it as an ideal laboratory in experimental evolution over a 

 lapse of seventy million years, and the record is remarkably clear. By 

 reference to the chart of Fig. 43.7, it will be seen that a group of early 

 immigrants were left isolated and proceeded to evolve in their own way. 

 The connection between North and South America to permit the influx 

 of these early mammals evidently was the result of volcanism and the 

 disturbances just reviewed. 



In late Eocene and early Oligocene time, shallow seas and volcanic is- 

 lands in the area of the isthmian uplift allowed certain forms adequately 

 equipped to make passage from island to island, and thus a wave of "is- 

 land hoppers" entered South America. After certain adjustments with the 

 ancient immigrants already there, the newcomers also proceeded to evolve 

 their own way. No continuous land of any breadth or durability was es- 

 tablished at this time, and the migration route served as a screen or sieve 

 to a host of North American forms which would have moved in under 

 more suitable environmental conditions. 



Orogeny and extensive volcanism again convulsed the isthmian swell 

 in late Tertiary and Quaternary time, and at first the region seems to have 

 been a chain of islands permitting a second wave of "late island hoppers," 

 and then a solid subaerial connection, permitting a wave of new immi- 



