These may be siliceous deposits or calcareous deposits. The sands are underlain by calcareous hardpan. On continental slopes and ridges calcareous silts are found. Usually on calcareous, or alkaline soils. The skeleton of sponges consists of microscopic calcareous or silicious spicules. It is often found growing in calcareous marl soil. The suborder Scleraxonia species are supported by a skeleton of tightly grouped calcareous spicules. They are derived from calcareous sediments of different degrees of purity. The walls of this have chitinous ridges, teeth and calcareous ossicles. It is blanketed in a thin covering of thick pelagic calcareous ooze. It is well adapted to calcareous soils. It is also in an area of black calcareous chernozemic soils. Most reptiles lay amniotic eggs covered with leathery or calcareous shells. The surface is undulating, and the soil calcareous and fertile. Alsatian soil being more dominately calcareous than the slate composition of Rheingau.