It is the bold simplicities of architectural form which seem to have held the most instinctive appeal for Cotman. When - in deliberate attempts to gain greater recognition for his work - he sometimes became overembroiled in intricate detail, his drawings, paintings or etchings of architecture, though admirably crisp, can seem oddly frigid. The impressive ''Durham Cathedral,'' however, falls squarely into the period now considered by many to be his most fulfilled, the period dominated by the so-called ''Greta'' watercolours. These are paintings he produced in a very individual style, mainly resulting from studies he made on his third (and final) visit to Yorkshire, in 1805. He worked near Rokeby, in the woodland bordering the River Greta.
An ordered tranquillity settles on these watercolours, a vision of classical balance and a tendency to perceive even such burgeoning and freely haphazard phenomena as tree foliage in terms of delineated masses of tone, of light and dark shapes in carefully placed contrast. It is as if the procedures involved in planning and composing a watercolour painting, the thoughtful locating of transparency or solid, of shadow or light, the overlaying of paint washes to build a composition, were Cotman's way of making sense of the visible world. Art was his rational frame of reference. Watercolour as a medium offers large freedoms, and demands in consequence severe restraints: the marvel of the ''Greta'' watercolours is that these two apparently contradictory qualities are in harmony rather than conflict.
The teeming tree-clothed banks of the river brought out in this artist a very evident enjoyment of natural exuberance. The ''stylization'' of this in his art may be achieved by lucid organization, by generalized shapes and by a restriction of his palette which harks back to the quiet, thin world of eighteenth-century English watercolour: but his romantic delight in nature, in its lush riches, its full measure, its untempered excitement and overspill, is far from lost and is even intensified. These meditative, self-controlled paintings are at the same time full of a young man's thoroughgoing praise for the immediate glories of natural growth.
He shared his enthusiasm for the Greta with the Cholmeley family, his patrons and close friends in Yorkshire. Mrs. Cholmeley in particular almost treated him like a son, and he developed a lifelong friendship with her actual son, who was apparently staying at Rokeby with Cotman when he was painting by the Greta. Mrs. Cholmeley wrote to them describing a visit to Castle Eden Dean, eleven miles from Durham: ''Give this letter to Cotty,'' she said, ''who will probably value it more than yourself.'' Castle Eden Dean reminded her of the Greta. It had ''far surpassed her most sanguine expectations.''
''Immense rocks,'' she wrote, ''rise on either side, sometimes abrupt, sometimes receding. . . . Fine ash, oak and elm flourish tho' almost apparently growing out of the rock itself, which rocks are sometimes clothed with festoons of ivy, sometimes with thick dark mosses of yew (which grow there in great abundance) and sometimes presenting a bold naked surface, seen thro' the light airy foliage of the ash.''
She managed by such enthusiastic descriptions to persuade her protege to leave the Greta for a short time in September 1805, to visit Durham (with a side trip to Castle Eden Dean, which he liked as much as she did, and painted) though he did not go to the city without a protest: ''Am I to place it on my studies of trees like a Rookery?''
It has been pointed out by Adele M. Holcomb that his large watercolour of Durham Cathedral (the finest of at least five views he painted of it) ''indeed rests on a pedestal of massed foliage.''
Like the Greta pictures, this fine work is a balancing of tones, of light against dark, dark against light. The darkening of the upper surfaces of the West Towers by cloud shadows has been pointed out as an indication of Cotman's feeling for ''atmospheric refinement.'' It is also a subtle way of making a distinction between the different styles of these towers - the lower parts being Norman, the upper being early Gothic. The tall central tower is also distinguished by a deeper tone frOm other sections of this magnificent building.
The painting belongs to the ''Greta'' sensibility, even though it is more a study of architecture than of nature. Cotman painted it as if he couldn't help seeing his Yorkshire vision wherever he went. Rivers flowing through the density of late summer trees had the same meaning for him whether in Durham or Yorkshire , and he saw the towering architectural monument growing upwards, with mountainous authority, as though it were a natural extension of its setting. What could have been better calculated to stir Cotman's particular kind of imagination at this time than such an inspired unity of man-built ''art'' with the exuberant abundance of ''nature''?