Now, limestones are not formed in the same way in which we have seen that sandstones and shales are formed. The last two mentioned owe their origin to their deposition as sediment in seas, estuaries or lakes, but the masses of limestone which are found in the various geological formations owe their origin to causes other than that of sedimentary deposition. In carboniferous times there lived numberless creatures which we know nowadays as _encrinites_. These, when growing, were fixed to the bed of the ocean, and extended upward in the shape of pliant stems composed of limestone joints or plates; the stem of each encrinite then expanded at the top in the shape of a gorgeous and graceful starfish, possessed of numberless and lengthy arms. These encrinites grew in such profusion that after death, when the plates of which their stems consisted, became loosened and scattered over the bed of the sea, they accumulated and formed solid beds of limestone. Besides the encrinites, there were of course other creatures which were able to create the hard parts of their structures by withdrawing lime from the sea, such as _foraminifera_, shell-fish, and especially corals, so that all these assisted after death in the accumulation of beds of limestone where they had grown and lived. [Illustration: FIG. 20.--Encrinite.] [Illustration: FIG. 21.--Encrinital limestone.] There is one peculiarity in connection with the habitats of the encrinites and corals which goes some distance in supplying us with a useful clue as to the conditions under which this portion of the carboniferous formation was formed. These creatures find it a difficult matter, as a rule, to live and secrete their calcareous skeleton in any water but that which is clear, and free from muddy or sandy sediment. They are therefore not found, generally speaking, where the other deposits which we have considered, are forming, and, as these are always found near the coasts, it follows that the habitats of the creatures referred to must be far out at sea where no muddy sediments, borne by rivers, can reach them. We can therefore safely come to the conclusion that the large masses of encrinital limestone, which attain such an enormous thickness in some places, especially in Ireland, have been formed far away from the land of the period; we can at the same time draw the conclusion that if we find the encrinites broken and snapped asunder, and the limestone deposits becoming impure through being mingled with a proportion of clayey or sandy deposits, that we are approaching a coast-line where perhaps a river opened out, and where it destroyed the growth of encrinites, mixing with their dead remains the sedimentary dêbris of the land. [Illustration: FIG. 22.--Encrinites: various. Mountain limestone.]