304 Mr. J. Orton on a Glacial Epoch at the Equator. 



The glaciation of the whole earth at the same time is absurd, 

 on biological and hydrological grounds : if, therefore, an 

 equatorial ice-period occurred before or after the ice-period of 

 the high latitudes, we must imagine the temperate regions 

 converted bj a change of climate into a conservatory for the 

 rich and peculiar life in the tropics — which is an unwarrantable 

 assumption. Polar types are now living in the intertropical 

 oceanic area ; so that their occurrence in any marine deposit 

 is no evidence per se of the general extension of glacial action 

 into tropical regions. And we may add that the almost total 

 absence of typical North- American plants in the highlands of 

 the West Indies and on the Andes of the equator does not 

 favour the theory of a glacial migration. 



No continent has such a simple geological structure as 

 South America. The monotony of its vast expanses is in 

 strong contrast with the complexity of Europe : witness the 

 unparalleled extension of gneissic rocks from the Orinoco to 

 Paraguay, the long, compact range of the Andes, so emi- 

 nently porphyritic, and the extraordinary continuity and 

 uniformity of the Llanos, Amazon, and Pampa deposits of 

 ochraceous sandy clay. Yet we have much to learn before it 

 will be wise to speculate on the geological history of South 

 America. Darwin and Hopkins have given us sections across 

 the Cordilleras ; and it is much to be regretted that Professor 

 Hartt has failed to give us a physical map, with geological 

 sections and reliable altitudes. We need a careful section 

 from Rio to Para, and another from Manaos to the mouth of 

 the Orinoco. Barometrical measurements are indispensable ; 

 but, so far as we know, the only consecutive observations 

 with a mercurial barometer across the continent are those 

 made by the writer in 1867*. 



It is probably safe to say this much : — that South America 

 began with the tablelands of Guiana and Brazil t ; that the 

 subsequent upheaval of the Andes left estuary friths now 

 marked by the three river-systems J ; that the Andes did not 

 reach their present altitude until after the deposition of the 

 Amazon formation, though it was a slow movement in mass, 

 for the beds are nowhere unequally tilted or dislocated § ; that 



* Published in the ' American Journal of Science,' Sept. 1868. 



t Bates has shown that the geographical distribution of insects indi- 

 cates that Guiana was formerly an island. 



X The sediments from these straits near the ocean would have a purely 

 marine character ; and Hartt observes that the clays and sandstones on 

 the coast tie in with those of the Amazon. 



§ This certainly follows, if the Pebas and Pichaua shells prove to be 

 early Tertiarj'. The clay -beds ascend the eastern slope beyond the village 

 of Napo, which stands 1400 feet above Para, and in long. 77°. The red 



