102 F. Chapman—On Sheared Oolitic Limestones. 
Fig. la is from a specimen which has been boiled in HCl before 
preparing the microscopic section from it, so that only the silica 
and iron pyrites are left. It will be seen that the spaces between 
the grains, and occasionally the centres of the latter are dissolved 
out, showing that a very small proportion of the original rock exists 
in this oolite. 
Fig. 2. represents a section taken from a slightly cleaved lime- 
stone, and in a direction vertical to the cleavage, from Watermouth 
Cave. In this example we have the first definite appearance of 
shearing in the rock. At this stage the grains have only suffered 
slight flattening, without much distortion, but irregular fissures 
have been made in the mass, passing through both grains and matrix 
in sinuous, but roughly parallel, lines. This specimen is also more 
or less silicified. 
At the next stage, shown in Fig. 3, there has been a great change 
in the structure of the rock-mass; it has been subjected to such 
pressure as to have caused the oolitic grains to be lengthened out 
to a remarkable degree, and to closely overlie one another. The 
length of the grains in this condition is nearly twice the probable 
original diameter, and the proportionate length and breadth as 
5 to l. A siliceous fringe is seen bordering these lengthened grains, 
when viewed between crossed nicols, but the silicification does not 
apparently affect the grains themselves, since a section, decalcified 
upon the slide, which had been cut from this rock, exhibited after 
the treatment dolomite crystals with a little silica around the pre- 
cincts of the grains, but showing that the grains themselves had 
been dissolved out. It therefore appears evident that the silicifica- 
tion of the limestones has only happened in the case of those 
oolitic rocks which have escaped shearing, or which have only been 
slightly acted upon by pressure. In the lengthened-out grains of 
this specimen, the concentric layers of the oolitic grains can be 
distinctly seen, as can also the radial structure which is so charac- 
teristic of the older oolitic rocks. The dolomite crystals occur in 
lenticular strings, the centre of each lenticle being occupied by 
granular material (perhaps ankerite), whilst on the outer margin are 
very well-developed crystals of dolomite often showing zone-structure. 
Seen under a high power these dolomite crystals show inclusions of 
very minute particles of iron-pyrites, and this latter mineral also 
occurs in some abundance in the fissures formed by the shearing of 
the limestone. 
In a hand-specimen this rock is of a dark blue-grey colour, with 
a distinct schistose structure; and traversing it, generally at right 
angles to the plane of schistosity, are numerous veins of pure white 
calcite, representing faults produced by the folding and crushing of 
the beds. 
An analysis of this dolomitized limestone has been made for me 
through the kindness of Prof. Thorpe, F.R.S., by Mr. Hrnest A. 
Pinchin, in the Chemical Laboratory of the Royal College of Science, 
and is as follows :— 
