14 
A TRIP TO ORKNEY AND SHETLAND. 
Jan., 1891. 
and it is considered that the nucleus of each of the “ tiles ” 
is really a nodule of ironstone, in various stages of forma¬ 
tion, or rather of concentration. Some of the specimens are 
argillaceous at the under side, and siliceous only at the upper 
face. 
The whole rock is an iron-coloured sandstone, and it is 
suggested that in the first place the surface became cracked 
and cross-jointed by a process of shrinkage, and then each 
of the so-formed “ tiles” would be subjected to the action of 
surface water (the surface drainage of the sloping land), 
trickling down through the little channels of division between 
the tiles. There is found to be a tendency in such isolated 
patches of sandstone, uniformly impregnated with oxide of 
iron, for the iron to become gradually concentrated towards 
the middle of the patch, by some kind of molecular attraction, 
deserting the margin, which becomes consequently light 
coloured, and migrating towards the centre, which becomes 
dark-coloured and ultimatelv converted into a nodule of iron- 
1/ 
stone. Such is considered to be a frequent process of forma¬ 
tion of ironstone nodules, although the mode of action and 
the operating force are not yet understood. 
I am indebted for these suggestions to Dr. Lapworth, who 
has kindly examined these specimens, and was much interested 
with them. The further question of the mode of formation of 
the mouldings round the nodules, and particularly where 
there is a succession of two or more mouldings, offers much 
difficulty, and is one of much interest for consideration. It 
has to be noticed that in several of the specimens there is an 
appearance of a succession of layers, in the form of saucer¬ 
shaped hollows lying one within another, and having the 
edges of the layers cropping out round the margin in parallel 
lines, suggesting a possible connection with the parallel 
mouldings that surround the centre nucleus. 
Dr. Tilden, who has also kindly examined the specimens 
chemically, finds 4 ‘ they all contain carbonate of lime, though 
in very different proportions ; mere traces appear in the 
interior, but there is a good deal in the argillaceous material 
of which the lower part consists in most of the specimens. 
Under the microscope the shining specks look like bits of 
mica ; they are iridescent, and apparently weathered.” He 
suggests that “ possibly the rock originally consisted of a sort 
of marl containing some sand, that joints were formed in it by 
contraction or otherwise, and that the upper part of each 
lozenge-shaped mass thus produced has been deprived of its 
carbonate of lime and fine particles of clay by weathering 
action, leaving the sandy particles behind cemented by oxide 
of iron. The singular double furrow is a point of curious 
interest.” 
