THE BUILDING OF THE DERBYSHIRE LIMESTONE. 223 



in the drawing of the magnificent fossil specimen of Encrinus 

 lUiformis (Fig. 5) in the Derby Public Museum. 



That these animals were abundant in the sea in which the 

 Derbyshire limestone was formed is certain, for we find strata 

 hundreds of feet in thickness, made up almost entirely of their 

 calcareous skeletons. The fragmentary nature of the fossil 

 remains is explained when we remember that the numerous 

 ossicles are held together by organic tissues which, on the death 

 of the animal, decay, allowing the skeleton to fall in confusion on 

 the sea floor. Cases occur, however, where we get almost perfect 

 skeletons, as in the fossil Encrinus shown in Fig. 5. 



Thus, by the accumulation of the skeletal remains of countless 

 generations of Crinoids, have thick masses of limestone rock been 

 built up. In addition to the Crinoids, however, are abundant 

 remains of Corals and Molluscs (Brachiopoda, Gasteropoda, 

 Cephalopoda, Pteropoda, etc.) which sometimes prevail to the 

 exclusion of the Crinoids, and thus we get, sometimes in the same 

 section, crinoidal, coralline, and shelly limestone. In several 

 localities, also, as at Castleton, Millers Dale, and Ticknall, occur 

 beds composed largely of Foraminifera. 



Considerable interest attaches to the question of the nature and 

 origin of the silicious and other impurities which are found 

 disseminated throughout the Hmestone. A piece of limestone 

 treated with concentrated hydrochloric acid is found to yield an 

 insoluble residue, which proves, when examined microscopically, 

 to be of great interest. Mr. Wethered has studied the insoluble 

 residues obtained from the carboniferous limestone series at 

 Clifton, and finds them to consist mainly of minute fragments of 

 quartz, together with a smaller proportion of tourmaline, zircon, 

 and felspar. Many of the quartz fragments are crystalline, and 

 the crystals are observed to contain nuclei of detrital quartz. It 

 is a well-known fact that damaged crystals placed in a solution of 

 the same substance possess the power of repairing themselves, and 

 we may regard the rounded nuclei as water-worn crystals, which 

 have attracted silica from solution to again build up the 

 crystalline form. There is little doubt that the insoluble residues 



