42 GEOLOGY. 



Above, the islands are only mere banks or mud-bars, brought in by the tributaries and de- 

 posited according to the action of the currents of the river. Where the current is rapid and the 

 course of the tributary is short, as is the case between the Las Moras and San Pedro, islands are 

 not formed in the middle of the river ; here they occur only as long narrow stripes of half im- 

 mersed bars and banks below or above the mouth of the tributary that causes them. Sometimes 

 the force of the river keeps back the deposit, and causes the formation of the island within the 

 mouth of the tributary ; such a case is presented at the mouth of the Arroyo "Piedras Pintas." 



Where the water is sluggish in the river and a strong current in the tributary, the islands 

 are pushed further out from the mouth of the latter. 



Islands, rising to the height of the alluvial banks of the river, 20 to 30 feet above the water, 

 do not occur above Eagle Pass. 



Cazneau's Island, named in honor of an American gentleman who resided in this vicinity for 

 several years and cultivated it to some extent, is the first that is situated above the common 

 high-water marks Such, however, are rare between Eagle Pass and Laredo, and it is not until 

 reaching a point some ten miles above the latter place that another island occurs which would 

 be worthy of consideration for agricultural purposes. 



Below Laredo, the first island of this kind is " Isla de los Eancheros." With this com- 

 mences a series of about twelve, which the Mexicans cultivate, raising corn, tobacco, melons, 

 pumpkins, and other produce for domestic use. 



To facilitate the identification of these islands, the following names were given to them in the 

 topography of the survey: Mustang Island, Belvidere, Maj. Brown Island, Carriso, Melon, 

 Green Tassel, Patriarch, Cypress, Island of Last Eocks, Isla Los Ajuntas, Beaver, Sabine, and 

 Green Key ; the latter one being only a narrow flat strip, subject to inundations, ought rather 

 to be classified with the islands of the upper region. 



Among the islands here mentioned, Los Ajuntas, near Mier, is, in consequence of its size and 

 fertility, the most valuable. It is also the largest one of all, being 2| miles in length by 

 almost J of a mile wide ; its upper part is heavily timbered, whilst its lower is open, and con- 

 stantly under cultivation. 



At the mouth of the Salado, and thence downward, the shore and banks of the Rio Bravo 

 are of a softer character ; the former is more muddy and sometimes miry, while the latter, 

 disintegrating and washing down, lay the foundation for, and causes an increased growth of, 

 vegetation. The soft soil, with the luxuriant undergrowth, added much to the labors of our 

 surveying party. 



At the Island of the Last Rocks, shoals and reefs, obstructing the navigation, make their ap- 

 pearance for the last time in the bed of the river, though rocks are still seen on both sides along 

 the shore. If rocks do appear in the bed of the river further down, no danger is to be appre- 

 honded from them because of their depth below the surface. 



The lower part of the green sand belt undergoes still another change in its external appear- 

 ance ; this is quite perceptible at the mouth of the Rio Salado, and downwards continuously from 

 that point. The frequent occurrence of oyster banks and shoals, together with the external 

 appearance of the strata, seem to verify the inference that the regions here and below are of a 

 still later age than the green fsandstone belt. 



The Rio Alamo seems also to indicate a distinct line in the geological features of the country. 



