The Cousin Jacks were an important part of early California mining and have left a lasting imprint on many of the old Gold Rush towns.

From this web site:

http://www.co.el-dorado.ca.us/stories/tommyknockers.htm

Cornish miners from Cornwall, England were some of the best mining men in the business, they simply had years of experience at tunneling and mining that the 49er�s lacked. In the diggings they represented as close to a mining engineer as one could find. Their sudden success in the mines encouraged mine owners to ask if they knew of anyone else back home with mining experience who�d be interested in coming to work in the California mines. The common answer among the Cornish was� �Well, me cousin Jack over in Cornwall wouldst come could ye pay is boat ride.� So many Cornish miners made their way to California this way, that it wasn�t long before the Americans had taken to calling em� all� �Cousin Jacks.�