Does a picture interest us because it has an interesting subject or because it is painted in an interesting way? Those who are largely indifferent to painting will tend to the former, finding pictures of pleasant things pleasant and being suspicious of any oddnesses inflicted by the paint - on the contrary, a painting is expected merely to hold a mirror up to the world, the degree to which the artist has concealed his machinations being a clear barometer of his skill and industry (spiritually defunct times have a particular inclination towards this fetishisation of "craft", which is reflected in the recent vogue for photorealist art). But surely there can be indifferent paintings of interesting things, extraordinary paintings of wholly ordinary objects? The history of modernist art is the history of the latter, the fabric of everyday life being subjected to an increasingly strange series of transformations. The modern sublime, in other words, is rooted not in subject, but in style.
This is where "American Sublime", Tate Britain's epic survey of early American landscape painting, comes slightly unstuck. There's a wilful conflation of subject and style, of "new land" and "new art". To travel in 19th-century America was doubtless to be overwhelmed by the beauty and immensity of the place, but it by no means follows that the art of the time will be awe-inspiring to a similar degree. Great scenery, as the work of every holiday dauber bears witness, does not a great picture make. And to claim that there are painters here "masterly enough to embarrass" John Constable and J M W Turner, as did the London Evening Standard's Brian Sewell, is just perverse. Take Thomas Cole, for example, the Lancashire-born artist credited with founding a new, uniquely American way of picturing landscapes. The most cursory comparison of his Mountain Sunrise, Catskill (1826) with Constable's The Leaping Horse (1825) makes the former seem the work of an accomplished amateur, a notation rather than a recreation of sublimity. Constable is painting an old land in a wild new way, while Cole, his neat little brush strokes mimicking the Dutch landscapists of two centuries before, is doing the reverse.
Not that there is necessarily anything wrong with that. In the presence of exceptional natural beauty, it could be argued, all our energies are spent on simply bearing witness. The converse of this is in the cities and domesticated landscapes of western Europe, where the deformations of the avant-garde were needed to re-establish contact with the sublime (that beauty which, according to one 18th-century theorist, also excites a sense of holy terror). But the painters on display here hardly needed to be "masterly", it being pretty much sufficient just to record what they saw. So it is easy to enjoy, in the widescreen panoramas of Kensett, Bierstadt, Gifford and Church, the vision of an almost supernaturally empty wilderness. These are paintings that have both America's grandeur and its conservatism, with a few basic elements (the distant peak, the smouldering sunset, the lonely hut, the sky-reflecting lake) occurring time after time. The prevailing mood is one of vacancy and peace, and this is largely because the technique is a polite re-rendering of classical European traditions. Whereas Constable's oil sketches bring chaos to the tranquil fields of Essex, these artists impose calm on an environment that was unfamiliar and dangerous.
This is an exhibition, though, that saves the big guns until last. None of Church's faintly mawkish sunsets can quite prepare you for the splendour of the pictures he completed after his travels to South America and the frozen north. The Andes of Ecuador (1855), Cotopaxi (1862), Rainy Season in the Tropics (1866) and The Icebergs (1861) have a scale and inner glow that do indeed bring Turner to mind, sublimity of subject for once corresponding with sublimity of expression. Similar in majesty (and also much indebted to Turner) is the somewhat later work of Thomas Moran, whose thunderously cinematic Grand Canyon of the Colorado (1892) provides the show's finale. But before it ends, the parallels between these early American pictures and the moving ones that were to succeed them are made explicit. The Icebergs is shown here just as it was on its sensational New York debut in 1861, in a darkened room of its own and bordered with a red velvet curtain. Having first queued and then paid to see this single painting, the public gazed into a radiant virtual Arctic so convincing that, in the words of one observer, "we absolutely shivered before it". Almost a century and a half later, a certain Hollywood film was just as popular - the only difference being that the virtual bergs in Titanic were painted not on canvas, but with computers.
"American Sublime: landscape painting in the United States 1820-1880" is at Tate Britain, London SW1 (020 7887 8000), until 19 May