Avalonia

Avalonia was a microcontinent in the Palaeozoic era. Parts of this old microcontinent lie under south-west Great Britain, southern Ireland, and the eastern coast of North America. Bits of it are in Europe, Canada, and parts of the East coast of the United States. Avalonia is named after the Avalon Peninsula in Newfoundland.
Avalonia started as a chain of volcanoes on the northern margin of Gondwana. It became a drifting microcontinent. The Iapetus Ocean in front of it slowly shrank. 
Avalonia collided with other continents. It ended up in the middle of Pangaea. When Pangaea broke up, Avalonia's remains were divided by the rift which became the Atlantic Ocean.
That is how it came to be part in western Europe, and part in eastern North America.