In view of the attention which has been drawn of recent years to the mode of origin of the purer limestones, I have been led to prosecụte some inquiries under this head on the Carboniferous Limestones of the Avon Gorge, near Bristol. I have paid greatest attention to the chemical composition of the rocks, particularly the proportion of insoluble residue remaining after treatment with hydrochloric acid; at the same time I have not wholly neglected the fossil contents, although for information on this point I have mainly relied on other observers.