164 WILMORE : THE STRUCTURE OF SOME CRAVEN LIMESTONES. 
The structure of Gerna is not so manifest as in the other 
cases. There is one quarry, and a few rather poor exposures. 
In the quarry the dips are in accord with those of Worsaw and 
the other knolls to the west. The knoll is, however, covered 
with grass for the most part, and the small exposures seem 
to be mainly in the obscurely bedded limestone, and the dips 
are not easy to make out. On the survey map they are given 
as quaquaversal, and this would indicate the probability of 
some roll in the beds. The limestone at Gerna is exactly like 
that in the knolls already noticed. The hard unpromising 
limestone becomes easy to work for fossils when it has weathered 
and the matrix of calcareous mud is more or less dissolved away. 
I have got excellent fossils from the weathered and decomposed 
material. The same tufa and stalagmite are noticeable, and 
I have got perfect cubic crystals which have a core of pj^rites, 
but are converted into limonite on the outside. Here, as 
always, there is evidence of the effects of long-continued 
percolation of water containing oxygen, carbon dioxide, &c. 
In one joint I found a well-worn quartz pebble, just such as 
occurs in the grits. 
(/) Sykes knoll stands up as a long narrow peninsula. It 
consists in part of crinoidal debris, and in part of fine-grained, 
apparently almost unfossiliferous, limestone. Exposures are 
not very mmierous, but the dips on the whole seem to agree 
with the general direction in the other knolls. A fault runs 
alongside, and it is separated from Twiston knoll b}^ a river 
valley. 
(g) Twiston knoll is the last of the series. It contains 
very fine crinoidal debris, with occasional shells ; this is some- 
times obscurely bedded, but in places it is most clearly well- 
bedded. 
There is again a fine-textured, apparently unfossiliferous, 
limestone at the base of the strata forming the knoll. There 
is another fine-grained deposit higher in the series, and the ^ 
more obscurely stratified crinoidal and shell deposits seem to i 
be mterbedded. The dips are nearly all southwards. ] 
To sum up, the structures and structural relations common 
to this series of seven knolls are as follows :— (1) The occurrence 
