THE PATHFINDERS; The Saga of Exploration In Southern Africa. By Peter Becker. Viking. 282 pp. $17.95.

EARLY IN this book the author sets up one of those classic confrontations between "civilization" and "barbarism" that replayed themselves throughout the 18th and 19th centuries. The year was 1805, and a South African magistrate named Hendrik Van der Graaff was leading an exploratory trek from Cape Town to Bushmanland. Now he was face to face with the head of a brigand band, one Danster, who was demanding food, tobacco and alcohol.

"As Van der Graaf was 'convinced that they . . . would seize by force what they could not obtain by solicitation,' he was for a moment unsure of what he should say. His voice firm, and eyes narrowed, he declared that, as merely the barest necessities had been brougt for the trip, only he, Danster, could ge given something to eat and drink and a pipe to smoke . . ."

This, then, was the moment of truth. The outlaw blinked. "Danster looked incredulously at Van der Graaff, then, feigning humility, said he had decided against molesting the expedition."

Whence comes this fierce courage that explorers always seem to be summoning? The late South African historian Peter Becker suggests a fanaticism for discovery as its source. Time and again in The Pathfinders, the governing authorities of the Dutch East India Company try to clamp down on the would-be trekkers in fledgling South African outposts. "Stick to farming," the officials advise. Time and again, though, the colonists succumb to their wanderlust, pack up their wagons, yoke up oxen, hire Hottentot guides, and set off for the Great Karroo or the Kalahari. Having made their move, they are not about to be checkmated by booty- hunters.

Though better-known for his later discoveries in East Africa, David Livingstone is a South African case in point. Livingstone, Alan Moorehead tells us in The White Nile, "was a traveller and a nomad: he had to keep on . . . because he was one of those people who cannot bear not to look over the other side of the next hill." The trouble was, Peter Becker adds, he often dragged his pregnant wife up the hill with him.

"'Was it not enough that you lost one lovely babe, and scarcely saved the other, while the mother came home threatened with Paralysis,' he was challenged in a letter received from his mother-in-law, 'and will you again expose her and them in those sickly regions on an exploring expedition?'" The answer, in effect, was yes: along they went on a trek to Lake Ngami, in what is now Botswana. Before returning to the mission station, Mary Livingstone gave birth to another child. At last her husband came to his senses and shipped them off to Scotland before embarking on another extravagant journey.

OTHER THAN Livingstone, Becker's pathfinders will be unfamiliar to most readers. They include Samuel Daniell, the draftsman whose drawings of Namibian natives are invaluably accurate and lovely to look at; William Burchell, the horticulturist who capped off his South African explorations with the classification of 60,000 collected specimens and the publication of his meticulous journals; and Henri Lichtenstein, the physician who described the Bushmen's habits with thoroughly modern anthropological precision.

Consider, for example, Lichtenstein's harrowing account of one way to imbue an arrowhead with poison. "He watched in amazement as one of the hunters, coming upon a puff-adder, trod on its neck, 'press(ed) the head fast together with (his) fingers,' and bit it off. Having extracted the venom, the little hunter then applied it to some of his arrow tips." Elsewhere, Lichtenstein catalogues the gruesome effects of being stuck by a poisoned arrow, but these the reader will be spared.

The Bushmen have made their way into the Western consciousness by dint of their captivating click language and the film The Gods Must Be Crazy, but other tribes described by Becker are more striking, as bizarre as anything in Tarzan: the Fish People, the Crocodile People, the Wild Cat Hordes of Queen Mantatisi. A special place in the Annals of Despotism should be saved for Shaka, King of the Zulu, who had "no fewer than ten of his courtiers executed for offenses which, in the opinion of the trading party, were trivial. Some of the victims had their necks broken by experienced neck-twisters, and others their skulls shattered with long- shafted knobkerries."

For all of its parti-colored material, The Pathfinders is not consistently as exciting as these excerpts might indicate. The shortcoming lies in Becker's laconic style and failure to provide the telling generalization that might put all of his details int perspective. But the material is so flamboyant -- puff-adders and tse-tse flies, quicksands and mountains veined with alabaster, poisoned arrows and prides of lions -- that the adventuresome reader will be glad to trek with him anyway.