6.3.
Mull schist slates
NM286245 Iona Abbey Illus 39,40a and 40b
From the earliest times in all parts of Britain where stone slates were available they were likely to be used in preference to blue slates. Stone slates were used in Edinburgh in preference to blue slates at an early date. No doubt where a castle was intended, the strongest and heaviest material on the roof would have been preferred. Whilst this assumption can be disproved in some particular cases and it is very hard to find adequate evidence in support, a national overview supports this theory. One reason may well be a delay in an understanding of slatey cleavage. Another would be the common use of wooden pegs in former times. These would stop a slate from slipping down a roof slope but would not prevent wind uplift, whereas the weight of stone slates would have made it possible for even the most exposed and vulnerable roof to be secure over long periods. The ideal candidate to fill this role in the Inner Hebrides must be the heavy schist slate quarried at one time at the Ross of Mull. Said to be of Moinian schist, this source of roofing slates was reopened at the beginning of the 20th century to provide authentic slates for the re-roofing of the nave of Iona Cathedral. This photograph of some of the spare slates kept at Iona shows the large size and considerable thickness of these slates.