6i4 SCIENCE PROGRESS. 



Among the oldest of the Tertiary rocks are those which 

 succeed the Cretaceous in the valley of the Brave (the Rio 

 Grande) between Laredo and Guerrero. They consist of 

 sands and marls, and belong- to the Lower Eocene ; and 

 they are succeeded by marly sandstones which belong in 

 part to the Eocene and in part to the Miocene. 



In the Peninsula of California, especially upon the 

 Pacific side, the coast is formed of sandstones and con- 

 glomerates, which consist largely of fragments of trachyte 

 and andesite. These belong to the Upper Miocene and 

 Pliocene. 



In the low-lying country along the coast of the Gulf of 

 Mexico, the Tertiary deposits form a band parallel to the 

 shore, extending from the Rio Grande southwards beyond 

 Vera Cruz, where they expand into a broad mass covering 

 the greater part of the states of Tabasco, Campechi, and 

 Yucatan. These deposits are marine in character and 

 consist of shelly limestones and agglomerates. In the 

 lower beds Miocene fossils predominate, while in the upper 

 beds Pliocene and living forms are common. 



Besides these, there are other deposits which must be 

 referred to the Tertiary period. Thus, the lignite-bearing 

 beds of Zacualtipan, in the state of Hidalgo, may be placed 

 at the top of the Miocene or in the Pliocene ; and numerous 

 conglomerates formed of Cretaceous and other fragments 

 are believed to be of Pliocene age. 



But the most interesting of the Tertiary rocks are the 

 volcanic lavas, agglomerates and ashes which cover so much 

 of the country. The period of vulcanicity began with the 

 ejection of syenites, hornblendic diorites, quartz diorites, 

 and diabases, the age of which cannot be exactly de- 

 termined, but is believed to be either late Cretaceous or 

 early Eocene. 



The next eruptions, which belong to the Miocene period, 

 consist of andesitic porphyrites, and propylitic andesites ; 

 and then followed, in order, hornblendic andesites, hypers- 

 thene andesites, augite andesites, and labradorites. The 

 last of these are evidently the precursors of the great 

 eruptions of basalt, which began at the close of the 



