island vs solid ground

island and solid ground both are nouns.

Word	Noun	Adjective	Verb	Adverb
island	Yes	No	No	No
solid ground	Yes	No	No	No

As nouns, solid ground is a hypernym of island; that is, solid ground is a word with a broader meaning than island:

Other hypernyms of island include dry land, earth, ground, land, terra firma.

island (noun)	solid ground (noun)
a zone or area resembling an island	the solid part of the earth's surface
a land mass (smaller than a continent) that is surrounded by water