116 GEOLOGY AND PALAEONTOLOGY. 



"Conglomerate, moutli of Guadaloupe river," breccia, probably of volcanic origin. 



"Santa Cruz Pass," compactly granular feldspathic rock, containing minute grains of mag- 

 netic oxide of iron. Another si)ecimen is of vesicular trap tuff, colored red by oxide of iron. 



"Rio Santa Cruz, above Tilbac," semi-indurated volcanic ash; also a specimen of coarse 

 breccia. 



" Igneous rocks west of Salado" — one specimen is compact, feldspathic lava ; the other com- 

 pact, coarse breccia. 



" Laguna Santa Maria," finely vesicular trap, with siliceous incrustation. 



"Twenty-five miles southwest of Frontera," highly vesicular trap or amygdaloid. 



" Baranca," a granular amygdaloid. 



The enumeration of this collection of specimens enables us to deduce some general conclu- 

 sions of great interest regarding the geological structure of the country between the Gulf of 

 Mexico and the Cordilleras range of mountains. The specimens ffom the eastern part of this 

 range, taken in connexion with what we know of its character in other places, and of the 

 geology between this range and the Pacific ocean, are sufficient to give a very correct idea of the 

 intervening space. 



A broad belt along the coast of the Gulf is occupied by deposits of very modern geological 

 age, which may be referred to the same period as the drift and alluvium. This deposit consists 

 of water-worn materials — as sand, gravel, pebbles, &c., which have been spread over the sur- 

 face in a very regular and even manner. The general elevation of this belt is 300 feet above 

 tide water, and varies little in its height for many miles in extent. 



In several jilaces the denudation of this deposit discloses beneath it formations of the earlier 

 Tertiary period. Approaching the borders of the high table-land which commences at the head 

 of navigation on the rivers, the cretaceous formation appears at numerous points in the river 

 beds and banks, and elsewhere where the superficial accumulations are removed. 



From the commencement of the table-lands westward, the specimens show the occurrence of 

 a broad belt of the cretaceous formation, interrupted here and there by isolated dykes, or 

 mounds of trap, or other igneous rocks of modern age. Basins of Eocene, marine Tertiary, 

 likewise occur at intervals, resting upon the cretaceous beds. The specimens of the latter 

 formation consist of limestones, some of them extremely compact and dark, and others light 

 colored and friable. Various admixtures of these with more argillaceous matter, and greenish, 

 calcareous sands, sometimes partially indurated, are of frequent occurrence. The numerous 

 fossils collected from different localities leave no doubt in regard to the age of this formation.* 



In localities where the igneous rocks are protruded through the beds of this age, a greater or 

 less degree of metamorphism has taken place. Sometimes we find a partial or entire induration 

 of the contiguous masses, and often their metamorphism is so great as to render it difficult to 

 distinguish their age and relations from a simple examination of specimens. 



Towards the west the igneous rocks, which first appear in small, isolated knolls, gradually 



" I should not omit to notice in this place the very valuable and interesting work of Dr. F. Kcemer upon the fossils of 

 the chalk formation In Texas, " Kreidebildungen von Texas," &c. This gentleman passed more than two years in the United 

 States, a considerable part of which was spent in Texas. Previous to the publication of this work he had published a 

 description of that country, with a geological map, &c. 



The coUectious now under oonsideration, tliough for the most part made at a distance from the principal localities cited 

 by Dr. Roemer, correspond to a great extent with those described by him, and corroborate in the most satisfactory manner 

 bis views of the general geological structure of the country. 



