The earlier dated were embarrassed and short; gradually, however, they expanded into copious love-letters, foolish, as the age of the writer rendered natural, yet with touches here and there which I thought were borrowed from a more experienced source. Some of them struck me as singularly odd compounds of ardour and flatness; commencing in strong feeling, and concluding in the affected, wordy style that a schoolboy might use to a fancied, incorporeal sweetheart. Whether they satisfied Cathy I don’t know; but they appeared very worthless trash to me.

I remained under the wall and perused Miss Cathy’s affectionate composition. It was more simple and more eloquent than her cousin’s: very pretty and very silly. I shook my head, and went meditating into the house. (Wuthering Heights, Ch 21)

Also, I forget if this is confirmed somewhere later in the book, but the "compound of ardour and flatness" makes me think that Heathcliff wrote or made suggestions for part of the letters, since a marriage between Linton and Cathy was a big part of his plan. (edit to fix highlighting)