Deer Isle Granite

Avalonia

Deer Isle is not really in North America; it is not on Laurentia, the craton that makes up the core of the North America tectonic plate. Instead, it is on the Avalonia terrain which began as an island arc of volcanos 800 million years ago off the coast of what was Gondwana (and before that Rodinia) and what became Africa. As the Iapetus (father of Atlas) Ocean closed during the formation of Pangaea, Western Avalonia accreted to Laurentia and remained stuck to it after the break up of Pangaea and the opening of the Atlantic Ocean. (On the other side of the mid-Atlantic rift, Eastern Avalonia formed large parts of Ireland, England, Belgium, and the Netherlands, and smaller bits of France, Germany, and Poland.) 

Deer Isle Granite originated in the volcanos' magma bowls; because they were enormous, the granite cooled slowly over hundreds of millions of years allowing time for the minerals in the stone soup to find themselves and form very large crystals.

Crotch Island compared with other Deer Isle Granites

Commercially available "Deer Isle granite" is quarried at Crotch Island; the granite pictured above is an example.  In fact, it is a brand name; only granite from Crotch Island can be sold as "Deer Isle granite." But it isn't the only kind of granite found on Deer Isle; below is a cross-section of a granite cobble from Sand Beach on top of a slab of Crotch Island Granite. Even though Sand Beach is only about a mile away from Crotch Island; the crystals are larger and the feldspar is more vermillion than liver-colored.