How did Gibb explain the Shetland black dust?

In Gibb’s account, the reported evidence is a fall of black dust on Shetland in October 1755. It looked like lampblack, smelled strongly of sulphur, coated the faces and hands of people working in the fields, and blackened their linen^[1].

The proposed source was Hekla, about 500 to 600 miles away. Gibb says that, because the wind was southwesterly, people presumed the dust had come from there; he added that he thought this was “undoubtedly true”^[2]. That is his historical judgment, not independent modern verification of where the dust originated.