46 GEOLOGY. 



Among the regular clianges to which the bar is subject, the most important is that caused by 

 the annual rising of the river, which occurs during the months of July, August, and September. 

 By means of this increased fresh-water power the deposits are carried and kept further out at 

 sea, perhaps as much as a mile from the mouth. During the rest of the year the bar again 

 approaches the beach ; the oscillatory action of the salt water prevailing for this time, it is 

 scarcely half a mile off the mouth of the river. The hydrographical distances between the bar 

 and the mouth may be found on map No. 1, in the archives of the Dejiartment of the Interior. 



Besides these regular changes, others occur monthly and daily that depend on the prevailing 

 winds from the seaside, or meteorologic influences of the country above, not including many 

 accidental occurrences which usually escape the scrutiny of the common observers. Of the latter 

 kind are the depositions of drift-wood, or even wrecks and pieces of timber, which, after being 

 washed ashore and left for awhile, are apt to form the foundation of a small bank or bar. Other 

 more perceptible causes are seen in the stormy weather raging upon the sea and in the hurricanes 

 on land. 



These agencies, incessantly at work, all tend to one result, no matter how different their form 

 of action may be, that is, addition to the land. The ebb and flow of the tide, the current of the 

 river and sea, though often acting in opposite directions, only collect and again distribute the 

 various material brought together by both fresh and salt water along the beach, where is pre- 

 sented by these gradiAl accessions the now forming and youngest portion of the continent. 



The operations of all these forces may be altered or varied by extraordinary influences that 

 may occur at any time, as they have done in ages past, such as the change of river beds in the 

 alluvial regions, the closing of the mouths of the river, and the opening of other outlets. 



There is not much doubt that the Eio Bravo once had its main mouth far from its present 

 locality, no matter how powerful the descent of its waters to the sea must have been in all time. 



Boca Chica, as its name implies, may be considered, without question, as one of the outlets of 

 the Eio Bravo, which has thus contributed to the formation of Brazos Santiago and Point Isabel. 



The currents over the bar of Brazos Santiago pour in a channel of such force against the tide 

 as to corroborate the inference that a still greater fresh water flow than that of the Arroyo Colo- 

 rado, some 25 miles to the north, comes into this bay. A glance at the course of the lowest 

 portion of the Rio Bravo leads to a similar conclusion, for it has a decided inclination to the 

 northeast after reaching the lowest portion of the colluvial belt. 



It may bo that the persons who attempted several years ago to dig a canal from the third 

 bend (above its mouth) of the river, towards Boca Chica, or Brazos Santiago, may have had 

 similar ideas on this subject. 



We have thus dwelt almost two long on the formation of the bar ; but however inconsiderable 

 this matter may appear, it is still highly important to know the agents by which restless nature 

 is achieving its work, the more so as the very same powers, now augmenting the land from the 

 seaside, have, in all human probability, been contributing to this end for ages. It really seems 

 beyond question that these oscillatory motions of the waters with the sweej) of the gulf stream 

 have deposited during the various ages of the cretaceous and perhaps tertiary sea that sediment 

 along the foot of the bold Sierras, which at the present day form the western and northern limits 

 of the now cretaceous basin of the lower Rio Bravo. 



Taking also into consideration the parallelism of the easternmost of the Sierras of Mexico and 



