34 GEOLOGV. 



San Baba and Pedernales. Again, about fifteen miles due north of Fredericksburg isolated 

 granitic rocks bave been met with, among whicb is the " Enchanted rock," of popular renown. 



Also, between the Llano and San Saba granite protrudes through cretaceous strata ; and six- 

 teen miles north of Fredericksburg occurs a coarse-grained granite, consisting of flesh-colored 

 - feldspar, gray quartz, and some little black mica. 



In other places along the Llano very finely grained varieties of granite have been observed. 

 Granite, frequently interspersed with veins and fragments of a white quartz, also appears at 

 various points. Pieces of syenite, too, have been found along several of the tributaries of the 

 Llano. 



Besides these plutonic forms, trap-like rocks also seem to occur in many places of the country 

 referred to by Dr. Roemer. Again, this author received from twenty miles to the northeast of 

 San Antonio de Bexar pieces of a black basaltic rock, which protrudes in veins through the 

 cretaceous limestone strata. In this basalt, as component parts, are many minute crystals of a 

 white fossil, (glassy feldspar?) and also a dark, olivinish fossil. 



The geographical distribution of the rocks of which Dr. Roemer speaks permits only the con- 

 clusion that all the marks of plutonic or volcanic formation must belong to the same system, 

 which, traversing the upper limit of the more recent cretaceous strata in the valley of the Rio 

 Bravo, shows itself in the shape of the low basaltic hills mentioned as occurring at the crossing 

 of the Rio Frio, and at the heads of the rivers Leona and Las Moras. 



There is no doubt that this dyke continues its northeastern direction, accompanying as an out- 

 layer of the higher regions of the Guadalupe and Ozark mountainSj and thus probably crosses 

 the whole of Texas, and possibly Arkansas. 



METEORIC mON. * 



With regard to meteoric iron, to whicb Dr. Roemer refers in connexion with the plutonic 

 rocks, and of which he mentions a large specimen now preserved in Yale College, we have to 

 state that, besides magnetic iron ore, which is scattered in loose inniimerable pieces of every 

 shape and size over the whole surface of the cretaceous basin of the Rio Bravo, meteoric iron is 

 inown to exist about ninety miles northwest of Santa Rosa. An American resident of this 

 town, Dr. John Long, called my attention to a piece weighing some twentj'-five pounds, which 

 was then in the possession of a Mexican ; small pieces had been cut from it, and hammered out 

 without the aid of fire into some trifling articles. It is said that the whole surface of the area 

 (embracing about thirty acres of land) where the deposition of this valuable mineral occurs is 

 covered with blocks of it, of greater or less extent, some containing as much, and even more, 

 than thirty-six cubic feet. 



GREEN SAND WITH LIGNITE. 



The upper limits of that portion of the cretaceous basin, which consists chiefly of strata of 

 green sand, and the course of the volcanic dyke discussed above, seem rather to run parallel 

 than approach each other. 



So far as our observations extended, the main portion of the cretaceous basin, from Las Moras 

 to the vicinity of Reynosa, forma a belt of 380 to 400 miles in width. 



