Sir,—In Number IV. of these notes, that on the Crystalline Rocks of Upper Egypt, I have mentioned the apparent absence of limestone from the Laurentian series as seen at Assouan. I should have added, however, that some of the crumbling schists seen in the low land east of the railway cutting resembled very closely the calcareous schists associated with the Grenville bed of Limestone on the Ottawa River, and gave the impression that Crystalline Limestone might not improbably occur in that vicinity.