Confidence: I'm as sure as I can be.
Notes: It is producing isidia, not soredia. Commonly, as here, the isidia of P. saxatilis are white on the sides and brown at the tips and occur in clusters along the pseudocyphellae. In photographs they can look like soralia, leading to misidentification. My description of the isidia is not constant in what we call P. saxatilis and I look forward to the results of current DNA research at Edinburgh.