The second volume of McCarthy's Border Trilogy concerns a different set of characters and takes place around ten years before the action of the more highly regarded "All the Pretty Horses". If I'm not mistaken, what I characterized as the negative romance of the latter is all but gone from "The Crossing". Overall the second novel is far bleaker and more meditative/philosophical (at its worst, plodding and overly long). Though to be sure there are some pulse-pounding sequences in the best tradition of the Western genre.
The "crossing" in question is ostensibly that of the main character, teenager and vaquero Billy Parham, across the New Mexico / Mexico border. He sets out, without telling his family, to return a she-wolf he has caught near his father's farm to what is most likely her original habitat in a Mexican mountain range. He must also make subsequent crossings and re-crossings in the wake of the events this decision sets in motion. True to McCarthy's style and substance, Billy and later his younger brother Boyd wander a desolate, violent and improbable Mexico torn between cultures as well as between past and future. Along the way various people they meet give philosophical substance and stylistic embellishment to what is otherwise a taut, Spartan tale of young men out for justice and troubleshooting the various pitfalls of living on the road in an inhospitable land.
I would suggest however that the true notion of "crossing" at play in this novel pertains to the nigh-infinite distance between the particular beings who inhabit McCarthy's chaotic, violent world. Much is made in the sequence with the she-wolf of the imponderable difference separating her from her captor Billy - just as in "All the Pretty Horses", John Grady and other vaqueros frequently muse on the distance separating man from horse. The sequence with the wolf comprises only the novel's first section, however. What is most interesting is how McCarthy spins the allegory of the wolf into a world where human beings are separated from each other by a veritable chasm, between which words and gestures make paltry communion. That is perhaps the point: for all the distance between beings, there is a type of communion, fleeting and unsure, that may be struck. The crossing is tenuous and terrifying and without guarantee, but it may be risked nonetheless; witness Billy standing in the dog-fighting pit with the wolf, or Boyd in his silent understanding with the young Mexican girl who joins them on their journey.
All of this puts me in mind of the later Lyotard, about whom I'm writing a doctoral thesis. In his later writings, Lyotard pondered the possibility of passage between what appear to be incommensurable faculties, genres of discourse, etc. His writings on Malraux in particular stress the notion of communion, love and solidarity between wholly alien beings (i.e. between human beings, who are essentially wholly alien to each other). "The Crossing" is, among other things, a singular study of these seemingly impossible yet altogether possible passages.