The main feature of the geology of Drake's Island is the association of volcanic rocks with marine limestone of Mid-Devonian age. The Geological Survey memoir by Mr. W. A. E. Ussher attributes the discovery of the volcanic rocks to Mr. R. N. Worth. Mr. Ussher leaves their precise nature, “whether lavas or tuffs,” somewhat in doubt, but states that they are “most certainly contemporaneous”. As a matter of fact both lava and tuff occur, and are definitely distinguishable.
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