MARKER : NOTES ON NORTH OF ENGLAND ROCKS. 3OI 



two sets of strongly-marked cleavage-lines. These are doubtless 

 joints from the stems of crinoids, the structure being highly 

 characteristic of the joints and plates of the echinodermata. The 

 various grains and fragments are enclosed and compacted by 

 a mosaic consisting of irregular crystalline grains of calcite, in which 

 are imbedded a few small angular fragments of quartz. 



A specimen [ro88] from the so-called 'Cockle Quarry,' a little 

 farther south and on the opposite side of the road, is a less pure 

 limestone, and contains more of the little quartz fragments. The 

 oolitic character, however, is well seen in a part of our slide, and the 

 grains here are formed sometimes around a fragment of shell, some- 

 times around a rolled pellet of what must have been a calcareous 

 mud. In the other parts of the slide the pieces of shells and pellets 

 of mud are not covered by any coating of calcareous deposit. Some 

 of the shell-fragments appear to belong to Pecten ; others, with wavy 

 form, represent the little RhyncJwnella spinosa {crossii) so abundant 

 at this place, and a few cross-sections of its spines are also seen. 



Going a few miles farther north, to Sancton, we take a specimen 

 of a very typical oolite [1089]. Here even the occasional quartz- 

 granules are thickly coated with brown-stained carbonate of lime 

 and built up into oolitic grains, and the mud-pellets have invariably 

 undergone the same process. Some of the grains have apparently 

 no nucleus, and these may show a very evident radial structure like 

 the spokes of a wheel. Most of the oolitic grains, however, are 

 formed upon fragments of organisms, none of which have escaped 

 this deposit of material on their surface. The chambered tests of 

 foraminifera are not uncommon. The valves of both brachiopods 

 and lamelli branchs are well represented, the latter showing a feature 

 very characteristic of these shells : the carbonate of lime which 

 originally composed their mineral substance was in the form of 

 aragonite, and this has been replaced by the more stable form 

 calcite, in clear granular patches preserving the outline of the original 

 shell but nothing of its internal structure. The matrix in which all 

 these various grains are set is, as before, a mosaic of clear crystalline 

 calcite. 



One other specimen of the Millepore Oolite may be noticed. 

 This is taken from Westow near IMalton, where the formation is 

 known as the Whitwell Limestone [945]- Here may be recognised 

 the fragmentary remains of crinoids, entomostraca, and other 

 calcareous organisms, with one or two scraps of bone, all covered 

 with the usual concentric coats of iron-stained carbonate of lime to 

 form ovoid oolitic grains. Other grains have for nucleus an angular, 

 or more rarely a rounded, fragment of quartz, or, again, a rolled 



Oct. 189D. 



