At first the rumours seemed too bizarre to be true: Thomas Pynchon had written a non-cabbalistic historical novel, a book that was humanistic and even rollicking. Thomas Pynchon, whose Gravity's Rainbow two decades ago seemed the end-point of a strenuously enigmatic approach to literature a book that seemed not so much filled with meaning, singular, as booby-trapped with meanings, plural. A novel that was both conspiracy and cover-up, as frustrating as it was fascinating.

Mason & Dixon is certainly a historical novel, among other things. It tells the story of the two English surveyors, Charles Mason and Jeremiah Dixon, who were charged with marking the boundary between Pennsylvania and Maryland, and so resolving a dispute that had lasted 80 years, on the eve of the American War of Independence, which made the dispute, though not the boundary, obsolete in any case. They see 'something styling itself 'America' coming into being, ripening, like a Treeful of Cherries in a good summer, almost as one stands and watches.'

This is the Age of Reason, as the characters constantly remind us, and themselves. In Mason& Dixon, though, it isn't the sleep of reason that produces monsters, but its obsessive vigilance. It may be in human nature to want an agreed boundary, but there's something odd about imposing a mathematical construct, an absolute line, on a landscape that knows nothing of Euclid.

It's only a matter of time before Mason and Dixon's line goes through the middle of a house. The wife in this case decides to remain on the side of the house that is no longer Pennsylvania (where she counted as married). The husband chooses instead to move the building so as to avoid double taxes. They will roll the house downhill into Maryland. Drawing the line leads to moving house and contingent divorce, and people bow to an abstraction.

Surveying a line by astronomical observation may seem a rational enterprise, but both Mason and Dixon have sold horoscopes in their time. 'Astrology', as we read in the vigorous eighteenth-century pastiche in which the entire book is written, 'is Astronomy's wanton little sister, who goes out and sells herself that Astronomy may keep her virtue surely we have all done the Covent Garden turn.' Not only that, but Dixon learned his surveying from a mage called Emerson, who taught his pupils to fly above ley-lines, those old straight tracks that are not, perhaps, abstractions after all. Not all the sciences of the period were rational, and Augustan clarity somehow feeds the Gothic shadows.

It is characteristic of Pynchon, too, to go in contradictory directions, towards absolute authenticity and its opposite. The novel rambles in proper picaresque fashion, but makes use also of such modern techniques as cinematic cuts. Extremes of variation are standard here, and of language and plausibility of incident. Pynchon's familiarity with the period, on both sides of the Atlantic is at least 10 times what most people would deem necessary for the writing of a historical novel, but his use of his expertise is highly deceptive. History is first softly wooed, then briskly goosed.

Pynchon knows all about the brutal employers in the Golden Valley of Gloucestershire (Mason's background), and refers confidently to the different business practices of the dayon the Wear (Dixon's native area) and Tyneside. But for him, research isn't an end in itself, but a beginning. He takes pains, then makes mischief. His accuracy in every register is complicated by conscious, even conscientious, anachronism of every sort. Reading Mason &Dixon is like listening to eighteenth-century music on period instruments, then realising you are hearing blue notes aplenty, and samples from the charts.

Pynchon's manner is so convincing that you can find yourself wondering if the modern 'no such thing as a free lunch' might not actually derive from the Latin tag Mason uses at one point, Prandium gratis non est. Dixon's toast in a Virginia tavern 'To the pursuit of Happiness' is overheard by a tall red-headed youth (shorthand enough for Thomas Jefferson), who asks if he can use the phrase some time. The text is studded with anachronistic inventions, usually ascribed to Jesuits, Freemasons or rabbis, such as self-winding watches or coffee machines.

But when it comes to a supposed ritual greeting between members of a cabbalistic sect that coincides exactly with the Vulcan salute from Star Trek (the hand held to expose the crotch between third and forth finger, and the verbal formula 'Live long and prosper') or anautical-looking man with big forearms and one eye screwed up against his own tobacco-smoke translating a phrase from the Torah as 'I am that which I am' (spinach, anyone?), then we are entering the realm of intellectual vaudeville. Comedy pervades at every level, from silly wordplay ('Suture Self', 'Sirius Business', 'Dutch Ado About Nothing') to elaborate set-pieces. Pynchon's sense of humour is surprisingly broad for a writer seen ascerebral, and some of his inventions, such as a mechanical duck in love with a French cook, would give more pleasure at shorter length. He is so entertained by the idea of writing a 'bodice-ripper' that he can't be content with simple bodice-ripping, which has already happened in Mason's presence by page seven. On page 156, a young woman rips her own bodice in front of Dixon and threatens to blame him for it ('Or was it a Spontaneous Seam Separation, apt to happen to any bodice, really?'). Rebuffed, she reassembles the ripped bodice, as if the garment were fastened with prophetic Velcro.

This book has a subtle, self-subverting attitude to the past, its hindsight alternately rendered invisible and flagrant. If there is one aspect in which it flatters modern assumptions, it is in its presentation of colonialism. The Rev Cherrycoke, through whom the narrative is filtered, sees the Black Hole of Calcutta as a retaliatory metaphor of the 'continental Coercion' that is the British practice in India. In the Cape, Mason feels the presence coiled behind everything of 'the great Worm of Slavery'. To be sure, there is novelistic pretext for some enlightenment of attitude; the reverend has a counter-worldly set of priorities (though he seems casually heretical about most matters); Mason knows first hand from Gloucestershire the systematic oppression of the weak by the strong, and Dixon is a lapsed Quaker, if Quakers can lapse. Still, it seems to evade the deep questions if we pretend hatred of oppression is instinctive among people we like.

The secret of the book's construction is expounded at its midpoint, in a discussion between minor characters: Lamination. 'Lo, Lamination abounding... its purposes how dark, yet we have ever sought to produce these thin sheets innumerable, to spread a given Volume as close to pure Surface as possible, whilst on route discovering various new forms, the LeydenPile (rudimentary battery), decks of Playing-Cards, Contrivances which, like the lever or Pully, quite multiply the apparent forces, often unto disproportionate results.' Mr Cherry coke explicitly mentions books as examples of lamination ('thin layers of patterned Ink, alternating with other thin layers of compress'd paper', though the original subjects of discussion were the croissant ('spread, fold, beat, flat, spread, again and again, eh?') and samurai sword.

Whether Mason & Dixon most resembles a pack of cards, a battery or, indeed, a samurai pastry, it's clear that Pynchon's technique of laminating opposites research and fantasy, humanism and game-playing is what powers this mighty book.