across vs crosswise

across is not an adjective while crosswise is an adjective.

Word	Noun	Adjective	Verb	Adverb
across	No	No	No	Yes
crosswise	No	Yes	No	Yes

As adverbs, across and crosswise are synonyms defined as:

Other synonyms of across include crossways.

across (adjective)	crosswise (adjective)
	lying or extending across the length of a thing or in a cross direction
	in the shape of (a horizontal piece on) a cross

across (adverb)	crosswise (adverb)
transversely	transversely
to the opposite side	not in the intended manner