Over here in the Channel Islands a lot of the old housing stock was built of granite from local quarries. The mortar appears to be mud or a poor clay mixed with a little lime.   Internal wall plaster was beach sand mixed with lime slaked on site. Once dry a fine sand lime was applied. White washed and freshened up as necessary. These walls kept the rain out because of their thickness. Usually at least 400mm thick. Rain water would penetrate then evaporate due to the open matr