588 POPULAR SCIENCE MONTHLY. 



Althougli the geological water way was open during this period (more 

 recent than has generally been suggested), it was afterward closed 

 for a time, and the subsequent connection between the Gulf of Mex- 

 ico and the Pacific is the most interesting to us, as reaching down 

 almost to the historic period. The date of the older strait was prior 

 to the Lafayette epoch (about the close of the Pliocene or com- 

 mencement of the Pleistocene period), while that of the later one 

 obtained well on in the Pleistocene period. The separation of the 

 seas was in part effected by gentle warpings of the district, but the 

 low Cordilleras seem to have been further squeezed upward by a 

 movement referred to on a preceding page. Out of the floor of the 

 present divide, the rocky islands of the recent strait became prominent 

 knobs (such as one shown in Fig. 9). The newly formed isthmus was 

 only a mile or two across. Its once nearly level floor, composed of 

 soft, earthy sandstone, has since been rounded into a succession of 

 hummock-like hills by the subsequent action of rains and rills. This 

 feature is shown in Pigs. 8 and 9. For a time the narrow isthmus 

 was penetrated by a geological canal, one hundred and fifty feet 

 deep and less than a quarter of a mile wide, the features of which 



<i|l Praa" 



GEOLOGICAL 



Straits of Tehuantepec 



Tig. 7. — Map of the Isthmus of Tehuantepec. The unshaded portion represents the high 

 plateau region as indented by the coastal plains (shaded). 



are now perfectly preserved in the channel, about a mile long, which 

 dissects the hummocky divide of the Tehuantepec Isthmus. The 

 northern end of this canal is shown in Fig. 9. The floor of this 

 passageway is still covered with water- worn gravel, which was de- 

 posited by the former currents. Connected witli this late channel 

 and at the same level, gravel terraces mark the shores of the Gulf 



