1914-15.] Structure of the Chalk in the West of Scotland. 299 
Silica in globular form has been described by Dr G. J. Hinde.* The 
globules vary much in size, the smallest being only *0014 in diameter. 
They are translucent by transmitted light and opalescent by reflected light. 
Evidence that colloid silica in globular form once existed in these chalks is 
in the multitude of minute bodies which occur through the section. A few 
are hollow, but most of them have become filled with minute crystals, the 
nature of which is undetermined. These infilling crystals not infrequently 
destroy the regular spherical contour, and many of the bodies referred to 
would hardly be taken to represent an original globule. In Professor 
Judd’s specimen (O 3) some of the globules of colloid silica occur in their 
normal glassy condition, and the gradual alterations to the opaque stage 
can be followed by the eye. 
0 5. — Siliceous Chalk, Beinn-y-Hattan. This was originally quite a 
different deposit from that of 0 3. Globigerina and Textularia, often large 
bold specimens, with other forms easily distinguished, occur with some 
regularity all over the section. Many are casts in a now opaque material 
(? once glauconite). There are few shell fragments or spicules. I am in- 
clined to think that Radiolaria occur in this section also. 
R 4. — Gribun, Isle of Mull. The section is for the most part clear 
silica, but in one portion sponge spicules can be seen thickly packed. It 
is possible to make out a few Inoceramus prisms and Foraminifera. 
0 4. — Beinn-y-Hattan. The clear siliceous matrix is in this section 
clouded in places with terrigenous streaks which may indicate terrigenous 
material. Though as a whole more obscured than in the first two sections, 
Foraminifera and spicules are well shown in places. Long thin prisms of 
Inoceramus similar to those in 0 3 are present. These forms seem to 
preserve their identity, whilst other organisms have become obliterated. 
Small quartz grains are scattered through this section. 
R 3. — Carsaig, Isle of Mull. The matrix clouded with streaks of fine 
opaque matter. Few organisms can be made out except small fragments 
of Inoceramus prisms and a few spicules. Part of some Rotaline form of 
Foraminifera has been infilled with the opaque matter and is well seen. 
Viewed with crossed nicols variations in the crystalline texture suggest 
that the rock was once crowded with organic remains. Large coarse sand 
grains occur in one part of the section. ' 
White Limestone, Carsaig, Mull (from Mr Jukes-Browne). This rock 
appears to have consisted of definite fragments, some of which may have 
been shell, though their structure is now quite obliterated. There are, 
* “ Beds on Sponge Remains in the Upper and Lower Greensand,” Phil. Trans. Roy. Soc., 
clxxvi, 1885 (1886), p>. 426 et seq. 
