146 
THE CAMBRIAN VOLCANOES 
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The volcanic geology of St. David’s possesses a special interest inasmuch 
as it embraces a tolerably full development of various features which 
characterize the volcanic groups of later Palteozoic systems. Though the 
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rocks are chiefly tuffs, they include also sheets of lava, as w-ell as sills, dykes and 
bosses. They show a remarkable range in chemical composition from quite 
basic to highly acid materials. They present the amplest proofs of having 
been erupted and spread out over the sea-bottom, and they likewise aflFord 
