500 W. J. SOLLAS ON THE SILURIAN DISTRICT 
Notes on the Rocks. 
1. Wenlock Limestone.—The concretionary ferruginous bed which 
immediately covers over the band of Wenlock corals, exhibits to the 
naked eye a spherulitic or oolitic appearance, which is due to the 
presence of a vast number of little, smooth, shining, reddish-brown 
bodies, having the form of oblate or compressed prolate ellipsoids. 
These little bodies, which look like so many minute beans or lentils, 
are present in so large a proportion as to form a great part of the 
rock, and, from their general appearance and close association with a 
coral limestone, might be taken at first glance for ferruginous oolitic 
grains. Their real nature i is best made out by astudy under the micro- 
scope of thin slices of the rock containing them. The rock is not of 
the same consistence throughout, some portions being harder and more 
compact than others ; and these are the best adapted for shemg. The 
harder parts occur either as isolated nodules or as cake-lke masses 
adhering to 4 coral or the remains of some other calcareous organism, 
the greater hardness and compactness in both cases being probably 
due to an internal deposition of carbonate of lime, and the adherence 
in the latter case to the derivation of a part of this deposited carbonate 
from the calcareous structure on which the hard mass is based. An 
examination of the slices I have had prepared shows a matrix of clear 
transparent calcite or dolomite, or of brown opaque finely granular 
ferruginous material, usually full of dispersed minute angular frag- 
ments of calcite and quartz. Imbedded in the matrix are the little 
lentil-like bodies under consideration, and minute, often fragmentary 
remains of various calcareous organisms. The former consist of an 
outer envelope of black iron-oxide surrounding a rounded fragment 
of calcite of organic origin. The envelope is more or less red on its 
outer surface; and its thickness is very trifling. 
The enclosed calcite may consist of a joint or ossicle of an Encri- 
nite, a fragment of shell derived from a Lamellibranch, or more 
usually from a Brachiopod, such as Strophomena, or a piece of a Bryo- 
zoon; or it may be of uncertain nature, or, in rare cases, an oolitic 
spherule. The broken edges of the shell-fragments are neatly 
rounded off, as if worn by attrition; and the outer envelope covers 
them like an even coating of paint. The minute structure of the 
shell is generally perfectly preserved, the canals of the Brachiopod 
shells being infiltrated by light-yellowish or deep-brown oxide of 
iron. The Lamellibranch shells have sometimes been infested by a 
minute boring fungus or alga, in which case the burrows of these 
plants are also filled with ite infiltrated oxide. The Encrinite-joints 
exhibit very clearly their characteristic reticulate skeleton, which 
has usually been more or less completely injected by some compound 
of iron which now presents a lightish yellow or dark red colour by 
reflected light; with transmitted light, on the other hand, it is quite 
black, owing to its complete opacity. Fragments of Encrinital ske- 
letons are also amongst the commonest of those other organic con- 
stituents of the rock which are scattered through the matrix with- 
out any enveloping ferruginous coat. They then very frequently 
