Sir,—...  Detachments from the cavalry troop went out (from Mayaguez) in the afternoon of the 11th on both roads leading to Lares; but the left hand or westerly of these roads was followed only a short distance, information, thought to be reliable, having been received to the effect that the bulk of the enemy’s force had taken the more easterly road, on which the town of Maricao is situated.  This part of the force was reported as making fair headway, having only a pack-train as transportation.  Reports also came to brigade headquarters that Spanish troops in large numbers, coming from different places,—­including Aguadilla and Pepino,—­were concentrating to attack my command.  While not impressed with the accuracy of these reports, I had the outposts strengthened, and placed a field officer in charge of them.  A party from the outposts, sent to reconnoitre the Las Marias road, brought word on the afternoon of the 12th that the rear-guard of the Spanish was still within five miles of Mayaguez, and proceeding slowly.

[Illustration:  The Rosario River, near Hormigueros.]

I immediately determined to pursue and, if possible, to capture or destroy this force, and at first resolved to move out with the entire command.  On reflection, however, I realized that there were objections to such a course.  The city and surrounding country were in an unsettled and excited state, the latter swarming with guerillas, deserters, and bushwackers.  I had no accurate knowledge of the spirit, strength, and location of the enemy’s forces, supposed to be within easy reach of Mayaguez.  Then, too, the rest of my command, already worn down by the exhausting marches and operations beginning on the 9th, had been seriously broken in upon by heavy outpost duty and drenching rains, which latter had made their camp a veritable mud-hole.  Furthermore, the road to Lares, except for the first eight miles out, was said to be all but impassable for wheeled vehicles; and this reminded me that the major-general commanding had intimated that I might have to go to Lares by way of Aguadilla.  I therefore concluded to despatch a reconnoissance in force, under Lieutenant-Colonel Burke, Eleventh Infantry, to harass the enemy and to retard its progress in every way.  The detachment was made up of six companies of infantry and one platoon each of cavalry and artillery, and started at ten o’clock A.M. on August 12.  It was given ample transportation for its three days’ rations and the infantrymen’s packs.  It was therefore as mobile as it could be made without a pack-train.  Hindered by excessive heat, followed by heavy showers, it marched only to a point where the two roads, above mentioned, are joined by a cross-road,—­or about nine miles.  I did not hear from Colonel Burke during the night, as I had hoped to; and the remainder of my command had its wagons packed, and was preparing to pull out on the morning of the 13th, when a courier came to me from him with a report of the difficulties that had retarded his progress, and of the presence of a Spanish force near Las Marias, variously estimated at from 1,200 to 2,500.  This force, the colonel said, had taken up a defensive position; and he was moving toward it...