F.. Chapman—On Sheared Oolitic Limestones. 101 
and other crystals, which have taken place in the rocks of this 
district. 
Deseription,—The limestones which appear to be in their least- 
altered condition are some encrinite-limestones from the east side 
of Hele Strand, and the cliffs at Hagginton Beach; and also others 
formed entirely of Cyathophyllum cespitosum and many other species 
of Rugose corals, occurring in lenticular masses in the slates of 
Hagginton Hill. In these limestones, nevertheless, solitary quartz 
and dolomite crystals can be often seen, though the latter mineral 
makes its appearance.in any abundance only in those rocks which 
have been subjected to pressure. In the specimens obtained from 
the neighbourhood of Rillage, and on to Watermouth, the dolomite 
occurs well crystallized although it performs a subordinate part in 
forming the mass of the limestone; but nearer to Combe Martin the 
rocks become completely dolomitized, where here and there, in a 
microscopic section, one meets with a stem-joint of an encrinite or 
other organism which points to the origin of the mass. 
The oolitic limestone before mentioned occurs over Sampson’s 
Cave, which is about 300 yards east of Rillaze; but the rock was 
inaccessible zn situ, though tumbled blocks of it were obtained at 
the mouth of the cave at low water. The same kind of rock was 
also found in freshly broken blocks at Watermouth Cave, about three- 
_ quarters of a mile to the east of Sampson’s Cave. The oolitic rock 
is of a dark blue-grey colour, and the separate granules which 
weather out at the surface are quite black. Some of the specimens 
of this rock from Sampson’s Cave have not, apparently, been subjected 
to much pressure, though all that were obtained show a greater or 
less tendency to cleave. The granules, under the microscope, are 
seen to be formed upon a nucleus of a shell-fragment or sometimes a 
quartz-grain ; and they are perfectly spherical in the plane of cleavage 
(where such exists), and nearly so at right angles to it. The oolitic 
granules were found to be much harder than the surrounding matrix, 
when polishing a section of the rock on a snakestone; the granules 
standing out in relief. This hardness was found to be due to the 
almost entire replacement of the original grain by microcrystalline 
quartz. Fig. 1 shows the oolitic structure before the distortion of 
the granules ; the latter are circular in outline and #;inch (-°925 mm.) 
in diameter. Around the grains may be seen a transparent boundary 
or fringe with a ragged margin, which is an outgrowth of the 
pseudomorphous or secondary quartz replacing those grains. When 
the section of the rock is examined under a high-power (i.e. 
+ inch), innumerable crystals of iron pyrites can be seen, showing 
triangular, square, pentagonal, and hexagonal outlines, and occurring 
disseminated through the oolitic grains in isolated erystals, without 
any regard to the concentric structure of the former; and also in 
the matrix in patches and strings, filling up incipient rifts and 
cracks. The blue-grey colour of the limestone and the dark tint 
of the granules is due to this occurrence of the iron pyrites; and the 
mineral performs an interesting part in the subsequent alteration of 
the rock, as will presently be seen. 
