On the 10th of April, Colonel Cross, a deputy quarter-master-general mounted his horse and proceeded to ride, as usual, for exercise, but the night passed without his return, nor was his fate known until ten days after, when a skeleton, found on the plains, was identified as that of the unfortunate officer. The mode of his death or the names of his slayers have never been discovered. But it was generally reported and believed that he had been captured by the lawless band of Romano Falcon, a ranchero bandit, and, after being robbed of every thing valuable, was shot with a pistol by the robber captain. With a view to check the depredations of these guerillas, Lieutenants Dobbins of the third infantry, and Porter of the fourth,--two bold and hardy soldiers,--were authorised to scour the country with a body of picked men, and capture or destroy any such parties they might encounter. It appears that they separated in quest of the enemy, and that Lieutenant Porter at the head of his own detachment surprised an armed troop, numbering nearly one hundred and fifty, engaged in jerking beef. Upon the approach of our officer one of the Mexicans snapped a musket at him, a salutation which Lieutenant Porter returned by the discharge of his double barreled gun. Upon this the Mexicans fled to the screen of the chapparal. Porter took possession of the horses and blankets of the fugitives, and, mounting his men, started for head quarters. At this moment, however, the rain began to pour down with the violence that is only witnessed in tropical climates, and whilst the Lieutenant and his party were passing through a dense copse of chapparal they were fired on by the enemy from an ambush. Shot followed shot from the secret foe in rapid succession, but our unfortunate men were unable to sustain the contest, as their powder had been soaked by the sudden shower. They wisely retreated, therefore, to the chapparal, and, separating into three parties, found their way to camp; but the luckless Porter, having been wounded in the thigh, was seized by the Mexicans as soon as his men departed, and despatched with their knives whilst they shrieked and yelled over his mangled body like a band of infuriate demons. Acts like these, characteristic of the worst periods of border raids, denoted the approaching storm. The country east of the Rio Grande bristled with irregular troopers. It was unsafe to go beyond the hail of sentinels, and the peaceful aspect of nature which had charmed our men so greatly upon their arrival was changed for the stern alarums of war. By the joyous peals of the church bells, the shouts of acclamation, and the report of spies, we learned that General Ampudia had arrived in Matamoros, and that, some days later, he was followed by Arista, who immediately assumed the chief command and apprised General Taylor, in courteous terms, that he considered hostilities commenced and was resolved to prosecute them. * * * * *