The novel opens in India, where Mary's parents and caretakers die of cholera. India is, therefore, identified with disease and death. It is a distorting periphery from which the badly spoiled and orphaned Mary must be rescued.
As the novel progresses and Mary becomes more acclimated to her new life in England, meeting Colin, her memories and stories of an exoticized India are ways she can relate to him in his "diseased" state. For example, she tells him:
Once in India I saw a boy who was a Rajah. He had rubies and emeralds and diamonds stuck all over him. He spoke to his people just as you spoke to Martha. Everybody had to do everything he told them—in a minute.
Colin's ill health, both physical and psychological (ordering the servants around imperiously), connects in Mary's mind to India. He must be cured of his "exotic" India-esque ways to find healing, health, and new life as a truly English boy.
At first, Mary is put off by the moors she sees in the darkness during the drive to her new home in England. Her lack of knowledge of what a moor is stands in for how alien and frightening England initially is to her:
Mary felt as if the drive would never come to an end and that the wide, bleak moor was a wide expanse of black ocean through which she was passing on a strip of dry land.
Mary at first tells her servant Martha that she "hates" the moor. Martha, who is not abject and who Mary feels she could not get away with slapping as she could her Indian servants, is identified with the moor, because that is where her family lives. If India is an exotic and "other" periphery that must be abandoned, the moor stands for England and the sturdy self-esteem of the English spirit as exemplified in people like Martha.
As time goes on, Mary begins to warm up to the moor, which represents her acclimating to her new life:
The rainstorm had ended and the gray mist and clouds had been swept away in the night by the wind. The wind itself had ceased and a brilliant, deep blue sky arched high over the moorland. Never, never had Mary dreamed of a sky so blue. In India skies were hot and blazing; this was of a deep cool blue which almost seemed to sparkle like the waters of some lovely bottomless lake, and here and there, high, high in the arched blueness floated small clouds of snow-white fleece.
In sum, the moor is a periphery associated with health and wholesome Englishness, while India is a periphery associated with disease and a dangerous otherness.