 Darwin saw Quail Island as his key to understanding the structure of St. Jago, and made careful studies of its stratigraphy in the way he had learnt from Adam Sedgwick.He collected specimens and described a white layer of hard white rock formed from crushed coral and seashells lying between layers of black volcanic rocks, and noted a similar white layer running horizontally in the black cliffs of St. Jago at above sea level.The seashells were, as far as he could tell, "the same as those of present day".