390 H. FOSTER BAIN 



vation terminaiing at the beginning of the Pliocene with the 

 emergence of the peninsula of Yucatan and all of the southern 

 part of the country which at the beginning of the Cenozoic had 

 been buried beneath the waters of the two oceans. Upon the 

 Pacific coast the depression seems to have continued in the 

 Pliocene, so that Lower California was for a time cut off from 

 Upper California by a canal and thus formed an island. On the 

 Atlantic coast the Pliocene included a period of elevation suc- 

 ceeded by one of depression and in turn followed by an eleva- 

 tion, the latter continuing into the Quaternary. 



It is not possible to fix absolutely the age of all the Tertiary 

 beds. Those west of Laredo in the Rio Grande valley seem to 

 belong to the Timber Belt beds of the Lower Claiborne of Harris. 

 East of the same place are beds in part Eocene, and in part 

 Miocene, possibly Lafayette. In the peninsula of Lower Cali- 

 fornia, particularly along the Pacific coast, is a series of shallow 

 water deposits resting upon the trachytes, andesites and dacites 

 of the Eocene and containing pebbles of rocks erupted in the 

 Miocene with fossils of Pliocene character. Along the Gulf of 

 Mexico are Tertiary marine sediments made up of incoherent 

 shell conglomerates and compact limestones. In the upper por- 

 tion these contain fossils which in other parts of the continent 

 are Miocene, mixed with recent and Pliocene forms. In the 

 lower part the Miocene forms dominate. 



The Tertiary in Mexico, as in the western portion of the 

 United States, was a period of great eruptive activity. The wide 

 variety of rocks and the large masses extruded are equally aston- 

 ishing. Syenites, hornblende-diorites, quartz-diorites, diabases, 

 porphyritic andesites, mica-andesites, dacites, and basalts are 

 all present. With the andesites are trachytes, rhyolites and 

 obsidians, and many transition varieties are present throughout 

 the series. The eruptions begun in the Tertiary have continued 

 to the present and have had much to do in shaping the topogra- 

 phy of the country. The whole of the eruptive rocks are 

 treated separately by Sr. Ezequiel Ordofiez. 



H. Foster Bain. 



