« Reply #1 on: Sunday 06 July 14 20:04 BST (UK) »
Looks like it may have Gaelic origins. Try genuki website and look for place names in Argyll. I think I remember seeing several names that started with Coalas.
Crackett, Cracket, Webb, Turner, Henderson, Murray, Carr, Stavers, Thornton, Oliver, Davis, Hall, Anderson, Atknin, Austin, Bainbridge, Beach, Bullman, Charlton, Chator, Corbett, Corsall, Coxon, Davis, Dinnin, Dow, Farside, Fitton, Garden, Geddes, Gowans, Harmsworth, Hedderweek, Heron, Hedley, Hunter, Ironside, Jameson, Johnson, Laidler, Leck, Mason, Miller, Milne, Nesbitt, Newton, Parkinson, Piery, Prudow, Reay, Reed, Read, Reid, Robinson, Ruddiman, Smith, Tait, Thompson, Watson, Wilson, Youn