FINE DISTURBANCES.(border patroling)

United States Border Patrol operations above the Texas stretch of the Rio Grande often begin with a single tracker on foot, staring at the earth. In the Border Patrol, tracking is called cutting sign. "Cutting" is looking; "sign" is evidence. No technology is involved. Trackers look for tread designs printed in the soil and any incidental turbulence from a footfall or moving body. They notice the scuff insignia of milling hesitation at a fence and the sudden absence of spiderwebs between mesquite branches and the lugubrious residue of leaked moisture at the base of broken cactus spines. (The dry time is a stopwatch.) The best trackers know whether scatterings of limestone pebbles have come off human feet or deer hooves. They particularly value shininess--a foot compresses the earth in one direction, which makes it shine, and wind quickly unsettles this uniformity, so high-shining groups are irresistible.

Low-light cameras and night-vision goggles and thermal-imaging scopes and seismic sensors are useful along the river, but in the brushland north of it pretty much the only thing to do is follow illegal immigrants on foot. Nearly every southern Border Patrol station maintains a network of footprint traps called drags--twelve-foot-wide swaths of dirt that are combed every day with bolted-together tractor tires ball-hitched to the back of an S.U.V. Agents monitor drags endlessly and follow foreign prints into the brush.

One morning just after dawn, I was out with an agent named Mike McCarson. He was driving a customized Ford F-250 down a ranch road, cutting a drag that ran alongside it. The Rio Grande was close; the sky was unusually congested. The drag and the ranch road extended out of sight through mesquite and prickly-pear cactus and purple sage and huisache and whitethorn, all semi-aridly dwarfish: the horizon was visible everywhere. Driving about eight m.p.h., McCarson leaned out the window a bit, his hinged sideview mirror folded flush with the door.

McCarson is forty-three; he has been tracking illegal immigrants for eighteen years. He works out of the Border Patrol's Brackettville station, which is about twenty miles northeast of the Rio Grande and thirty miles east of the town of Del Rio. Brackettville's hundred and twenty agents are responsible for a twenty-five-hundred-square-mile rectangle of mesquite flat and limestone breaks. No more than forty agents are ever in the field at once--one per ninety square miles.

Federal guidelines recommend that undocumented aliens be apprehended at least five times before they're charged with illegal entry and held for trial, but Brackettville's agents rarely detain people unless they've been caught more than fifteen times. On any given trip, an illegal immigrant's odds of eluding Brackettville's defenses are extremely good. Immigrants can angle in from anywhere along a fifty-mile stretch of the Rio Grande, and overpowering numbers of people often cross the river simultaneously. Brackettville doesn't have the personnel to track more than five groups at once.

"It's just the luck of the draw which ones we chase," McCarson told me as we drove. "We generally don't even work groups as small as three, four people, or groups that happen to have crossed real early the night before."

I spent many days with McCarson, cutting drags and trails and meandering through mesquite in the F-250. He is a big, sauntering, freewheelingly mouthy guy who says "tars" for "towers" and "boo-coo" for "beaucoup," a word he uses a lot, sometimes as a noun ("There's boo-coos of trails through there"). McCarson instinctively registers the happenstance, ambient comedy of ordinary life, sometimes with a falsetto, arc-of-joy cackle. He likes to stop for lunch on hillsides and in old Seminole cemeteries, and he can be precipitately melodramatic. ("Now, the seasoned journeyman agent will see the totality of circumstances in his area and know precisely what's normal activity and what's not.") McCarson is married but doesn't have kids, which may partly explain his widely entrepreneurial imagination: he owns a scuba business on Lake Amistad; he's a partner in a San Angelo real-estate venture; he's a successful self-taught stock picker.