PART X. CALCAREOUS GROUP OF BUILDING STONES. CHAPTER I. LIMESTONE. Limestone of several varieties is largely employed as a building material. These varieties depend very much on differences of origin and composition, and correspond to successive geological periods. Amongst the oldest formations, limestones are comparatively rare, at least in the British Islands and Europe, and it is not till we ascend into the Devonian, and es pecially the Carboniferous periods, that they assume a high importance as compared with the other strata. From this period forwards into Tertiary times they are proportionately more fully developed, till amongst the Cretaceous and Lower Tertiary formations, they become the most conspicuous members. This gradual augmentation in volume, as compared with the associated sedimentary strata consisting of various forms of sand or clay, appears to be intimately connected with the development of those classes of