Is of the mountains highlands an adjective phrase?

No, "of the mountains highlands" is not an adjective phrase. Here's why:

* It's a prepositional phrase. The phrase begins with the preposition "of" and modifies "highlands."

* It acts as an adjective, but it's not a phrase specifically called an adjective phrase. Adjective phrases typically consist of an adjective as the headword, followed by modifiers (adverbs, other adjectives, etc.).

Example of an adjective phrase: "very tall" (very is an adverb modifying the adjective "tall")

How to rephrase it as an adjective phrase:

You could use a more descriptive adjective to replace the prepositional phrase:

* "mountainous highlands"

* "highland mountains"

These options use adjectives directly to describe the highlands.