Did you know that at one time Island County was very large and comprised all of what would become Whatcom, Snohomish and San Juan counties? For a brief time, it was the second largest county in WA Territory.
On this date (January 14) in 1861, Snohomish County was created from the mainland portion of Island County.
For a detailed history of how Island County was created and then whittled down to its current size, read this interesting article on HistoryLink: