A two-wheeled progress through South Africa baffled the natives, says Christopher Hope; South From the Limpopo by Dervla Murphy John Murray, pounds 18.99
South Africa is a well of self-righteousness. Visitors taste its waters and come away feeling superior. Dervla Murphy was different to some degree - she did it on a bike. She visited South Africa on three occasions, twice before the 1994 elections and again after the vote that brought a democratic government. She rode from the Limpopo to the Cape and pedalled across the desert spaces of the Karoo into deepest Zululand.

She was barely across the border from Zimbabwe when the mining town of Messina hove into view, its verges strewn with beer bottles and plastic bags: all signs, to her distress, that South Africa is a "developed" country. It is also a very wicked country, once ruled by the Boers and now guided by Mandela, who has ranged against him the forces of Afrikaner reaction and villains such as Gatsha Buthelezi. So fierce is her eye for racial discrimination that she is no sooner in the country than she is drawing up an apartheid of the vegetable kingdom: oranges are native fruit; but black hawkers, she notes, also sell "such European vegetables as cauliflowers and carrots".

It is a remarkable progress. Like the Victorian explorers of the dark continent, Murphy has read her history and studied her geography, and she lets little interfere with her iron progress. And, like early missionaries, she is a source of constant bewilderment to the natives.

Does she intend to counsel them - or kill them? Or is she simply determined to kill herself? If so, will they be blamed? This crazy white woman on a pushbike forever gatecrashes their beer parties and chastises them for their customs. They greet her with astonishment and, I suspect, wave her on with relief: head high, panniers packed with ideological baggage, biking across the killing fields of Natal and the violent high veld. She is appalled by almost everything she sees.

Anyone with such firm views about vegetables will have a pretty sharp way with Afrikaners. Murphy spots her first group in Messina and falls quite naturally into the language of the field anthropologist, or the tourist studying game in a reserve. She marks the "aggressive body language" of Afrikaner males and notes that "the older male tends to be pot-bellied". The female of the species is no more attractive: her body language is "defensive-agressive" and her natural camouflage leaves a lot to be desired: "permed, laboriously made-up, fussily dressed". It is remarkable that those who attack Afrikaners for their perceived racialism feel free to talk about Afrikaners in much the same terms.

In time, she warmed to the Afrikaners she met and, in the end, is claiming that some of her best friends are Boers. The Boers may be beastly, but English-speaking whites are worse, hobbled by their "colonial snobbery".

She is reliable enough on hotel prices, and on finding the way to the bar - useful talents in the traveller. If she dislikes Afrikaner louts, she loves their lager. I have stayed in many of the same places: from tiny Vosburg, the Karoo village so proud of its police-station, to the eerie opulence of the Ulundi Holiday Inn, the only hotel in Buthelezi's capital; and in countless seedy hotels in woebegone country dorps. But an air of disapproval clouds every turn of the wheels.

What Murphy is after is not knowledge but repentance. It is not enough that the whites should come to terms with the changes now before them; what is required is punishment. Only at the end - on her third trip in December 1994, as the euphoria that accompanied the great change begins to ebb - is she given a single, terrifying glimpse of a truly South African reality. It comes when, determined as ever to ally herself with the incomprehending poor and the astonished oppressed, she spends Christmas with a family of black shack-dwellers in the shanty town of Khayelitsha, outside Cape Town.

It is a very South African Christmas. Her friends sponge off her shamelessly. On a family visit to the local jail where relatives are locked up for carrying illegal firearms, everyone, including toddlers, gets monumentally drunk. Christmas Day is Dervla's treat: she takes the family for a day at the beach, where the favourite attraction turns out to be watching young men knock a woman down and assault her by kicking her in the genitals.

Dervla Murphy's friend and host, Georgina, explains: "It's bad, I know it's bad. But always you get a buzz from it. I can't explain it, it's like an addiction, like brandy or mandrax." This final encounter is worth the rest of the book. It speaks of the way things truly are: of the terrible, compulsive violence knitted into the fabric of South African life. The nicest people do the most terrible things.

"There was," Dervla Murphy writes, "nothing more to be said." On the contrary: in these final, explosive pages, she says it all.