In Chapter 9, Catherine uses the similes of foliage and rocks to compare her love for Linton and her love for Heathcliff.
My love for Linton is like the foliage in the woods. Time will change it, I'm well aware, as winter changes the trees—my love for Heathcliff resembles the eternal rocks beneath—a source of little visible delight, but necessary. Nelly, I am Heathcliff—he's always, always in my mind—not as a pleasure [...] but, as my own being[.]
When Catherine says that her love for Linton is "like the foliage in the woods," she acknowledges that her love is real and that it's even beautiful, but also that it's subject to inevitable change; presumably, it could even go dormant or die off someday, as trees do in the winter. Her love for Heathcliff, on the other hand, is as solid and unchanging as the earth's foundations. Their relationship might not look lovely to outside observers, and it might not even be enjoyable for her, but it's the bedrock of Catherine's very existence. Catherine even goes so far as to identify herself with Heathcliff—he's so present in her consciousness that she can't conceive of herself without him.
The contrast between these two similes—beautiful but fading foliage and ugly but immovable rocks—sums up Catherine's contrasting feelings for the two men. She loves both Linton and Heathcliff, but only Heathcliff is "necessary" for her own being.
After Catherine dies, Isabella tells Nelly how Heathcliff has been behaving in his mad grief, ironically likening Heathcliff to a pious Christian:
There he has continued, praying like a methodist; only the deity he implored is senseless dust and ashes; and God, when addressed, was curiously confounded with his own black father! After concluding these precious orisons—and they lasted generally till he grew hoarse, and his voice was strangled in his throat—he would be off again; always straight down to the Grange!
Isabella recently married Heathcliff and has suffered from his abuse, telling Nelly earlier that she doesn't regard him as human. Here, she ironically twists the simile "praying like a methodist" around by alluding to Satan and suggesting that, far from being a prayerful, sympathetic mourner, Heathcliff is a monster and damned.
The Protestant religious movement known as Methodism had only become widespread in England by the mid-1700s; at this point in the story's timeline (the 1780s), "Methodist" was a broad-brush term that could be used to apply to anyone who was fervent in their religious practice, especially in enthusiastic prayer. But Isabella doesn't mean the term at all literally, as her next comments show.
The "deity" of "senseless dust and ashes" refers to the late Catherine. In other words, Heathcliff isn't even praying fervently to God, but to the spirit of his dead beloved. Further, "his own black father" refers to the devil, with whom Heathcliff, in his grief and rage, blasphemously conflates God (and Isabella calls Heathcliff the devil's offspring). The fact that Heathcliff's desperate "prayers" are a mashup of pleading and curses (none of which are directed to God) demonstrates that he is utterly irreligious. Like "methodist," then, "precious orisons" (or prayers) is verbal irony, sarcastically characterizing Heathcliff's blasphemous ravings as sincere piety.
Nelly reflects on Cathy's 16th birthday, when she and Cathy were out walking on the moors. She uses imagery conveying youth, promise, joy, and goodness:
She bounded before me, and returned to my side, and was off again like a young greyhound; and, at first, I found plenty of entertainment in listening to the larks singing far and near; and enjoying the sweet, warm sunshine; and watching her, my pet, and my delight, with her golden ringlets flying loose behind, and her bright cheek, as soft and pure in its bloom as a wild rose, and her eyes radiant with cloudless pleasure. She was a happy creature, and an angel, in those days. It’s a pity she could not be content.
Nelly describes Cathy as a "young greyhound," with boundless energy, and remarks affectionately on the young woman's carefree, innocent radiance—the "bright," "wild rose" complexion and "cloudless" happiness in her eyes. The natural environment reflects Cathy's joy, too, with singing larks and pleasant sunshine. Yet this lively, fruitful, promising imagery also foreshadows the loss of Cathy's innocence and freedom. Cathy is fresh and jubilant on the open moor, but she will soon be trapped, stagnant, and bitter as Heathcliff's prisoner at Wuthering Heights.
Given Nelly's perspective, looking back on this memory as she recounts it to Lockwood, the happy day on the moors has a wistful undertone ("She was a happy creature, and an angel [...] It's a pity she could not be content"). Nelly knows that Cathy met Heathcliff and Hareton on the moors that same day, which spelled the beginning of the end of her sheltered happiness. She implies that Cathy's encounter with the men of Wuthering Heights precipitated a loss of her "angelic" innocence, and that it stirred a latent discontentment in her. Her lively curiosity, then, ends up being her downfall, as she starts visiting the Heights and being pulled out of the protective world of the Grange that has fostered her innocence thus far.
Cathy and Nelly have gone to visit Linton on the moors, but Cathy's father is dying, and she hates to leave him just to keep a promise to sickly, peevish Linton, who's poor company. Nelly likens Cathy's expression to the surrounding landscape:
Catherine’s face was just like the landscape—shadows and sunshine flitting over it, in rapid succession; but the shadows rested longer and the sunshine was more transient, and her poor little heart reproached itself for even that passing forgetfulness of its cares.
The alternating shadows and sunshine suggest that Cathy feels conflicted over Linton. Though she likes having a potential love interest to flatter her and to boss around, she isn't really passionate about their relationship—at least, she cares about her father more than she cares about her whiny cousin.
More to the point, the landscape simile suggests Cathy is maturing. She's always been sheltered and carefree, but now, "the shadows rested longer and the sunshine was more transient," suggesting that with her father's imminent passing, she's assuming more adult cares that overshadow childlike joys. This image also foreshadows the darker turn Cathy's life will take after she marries Linton.
Finally, like her mother Catherine's, Cathy's beauty is associated with the wildness and beauty of the moors—her expression reflects the landscape's variable moods, and her heartaches and passions seem closely linked to her natural environment rather than artificially cultivated.