168 WILMORE : THE STRUCTURE OF SOME CRAVEN LIMESTONES. 
corals, and poJyzoa are identical in the Tiiornton and Rain 
Hall beds of blue limestone and muds and in the limestone 
knolls. The brachiopods are only different in that muddy 
water forms predominate in the one, whilst Spiriferid and Rhyn- 
chonnelid forms are plentiful in the other. The difference is 
quantitative and not qualitative. 
An apparent pecuharity of the knolls to which attention has 
heen very often drawn is their extremely fossihferous character. 
This is partly due to secondary causes. It is well to remember 
that some of the knoll limestones are apparentlj' unfossihferous 
until they are examined in thin section or until their iossils 
are " weathered out." There are some recent exposures in 
the Twiston knoll which are apparentlj^ quite unfossihferous, 
whereas all the weathered surfaces and all the old walls on and 
near that knoll show plenty of fossils. 
The shells and crinoids and other remains of organisms 
are held together by a fine calcareous cement. Long-continued 
weathering attacks this cementing matter, and reveals the 
megascopic fossils, and renders them quite easy to extract. 
Everyone who has worked on the limestones knows how^ beauti- 
fully fossiliferous are the old waUs, and how much better their 
western faces are than their eastern faces. It is the same in 
the joints of the knoll limestones. Masses which appear quite 
hopeless on newly fractured surfaces show excellent fossils of 
various types wiiere the bedding planes have been long ex- 
posed or where the joints are accessible. 
Now the dip of the knoll limestones is evidently very favour- 
able to the action of percolating water, wdth its dissolved gases 
and organic acids. In the more thinly bedded masses the 
action reveals fossils along the frequent joints and along 
the parting planes of the bedding, as has been already pointed 
out. In the more irregularly bedded limestones the meteoric 
waters have occasionally percolated throughout the whole 
mass, the cementing material has been partly dissolved aw^ay, 
the iron compounds converted into hydrates, the remainder of the 
cement converted into a yellow or brown day, and the shells and 
other fossils left so that the}^ can be lifted out with the fingers. 
