678 



GEOLOGY. 



of the material employed were once instinct with life and animation, immeasurable ages 

 before the creation of his race. A very rare crinoideal animal, the Pentacrinus Caput- 

 Medusa, fixed at the bottom of the seas of the Antilles, having a column formed of 

 numerous pentangular joints, is now the chief living representative of those mighty 

 swarms that were among the first inhabitants of the ocean, whose petrified bones form a 

 vast extent of strata in northern Europe and America, as essentially as a hayrick is 

 composed of grassy fibres. 



But among the organic remains of the Wenlock limestone, the corals are the most 

 numerous, and form the most striking characteristic of the stratum. They belong to 



1. Porites discoidea. 



WENI.OCK CORALS. 

 2. Porites petalliformis. 



3. Cyathophyllum caepltosum. 



upwards of fifty species, and exhibit structures of great beauty, as may be seen from 

 the accompanying engravings. It may here be observed, that there is a loose and inac- 

 curate mode of speaking of corals, as if they were formations apart from the coral 

 animals ; whereas the relation between the two is analogous to that of the bones and 

 flesh in the human frame* 



The Ludlow group. This division comprises the uppermost series of Silurians, of 

 which three subdivisions are made, the Aymestry limestone, intervening between the 

 Upper and Lower Ludlow rocks. The latter is a great argillaceous mass, varying in 

 colour from dark grey to black, forming the popular " mudstone " of the district ; in 

 other places called " waterstone,*' from its tendency to decomposition. Some of the 

 beds contain spheroidal concretions of compact earthy limestone formed around organic 

 remains. Mr. Murchison counted thirty fragments of the tribolite Asaphus caudatus in 

 a single nodule. The Aymestry limestone is sub-crystalline and argillaceous, yielding 

 a valuable lime for cement under water, named after the beautiful village where it 

 is most distinctly seem It appears at various places apart from this particular locality, 

 as on the east of iJerefordj where the Silurian strata protrude through the old red 

 sandstone ; and at Sedgeley, in Staffordshire, the rock presents itself through the car- 

 boniferous formation in a highly calcareous form. It is every where charged with 

 shells. One species, with a peculiar organisation, and almost exclusively confined to 

 this calcareous zone, has been named after a distinguished naturalist, T. A. Knight, 

 F. R. S., in whose domain at Downton Castle it is very abundant. The Upper Ludlow 



