Professor G. C. Gough—Metamorphism of Chalk. 147 
calcite showing well-marked lamellar twinning and rhombohedral 
cleavage. One section shows small flakes of mica which are probably 
phlogopite. 
Adjoining, and also caught up in, dyke is a rock which weathers 
so dark that it is difficult to tell it, except on a freshly fractured 
surface, from the dyke. It is found on microscopic examination to be 
the chalk completely altered into what may be termed a Cale-Silicate 
Hornstone, and specimens from different positions, while coming 
under this name, show different proportions of the minerals present. 
The size of the constituent crystals also varies from rather finely 
eranular ones, mutually interfering, to larger well-marked crystals 
which are in places idiomorphic. ‘The minerals found in the various 
specimens may be enumerated preparatory to a description of the rock. 
They are Wollastonite, Scapolite, Zoisite, Calcite, Diopside, Anorthite, 
and Magnetite. I strongly suspect Ilmenite as being present, but 
I could not prove it by chemical tests, while another mineral seen in 
sections as tiny, idiomorphic, octahedral, yellowish-brown, isotropic 
crystals is possibly Perofskite, but as I did not find it in the crushed 
rock I could not be certain of its identification. Garnets were 
specially looked for, but not detected, nor were any coloured minerals 
present such as Idocrase or Epidote. 
The identification of the various minerals in the sections is by no 
means easy, as with the exception of the magnetite they are clear and 
colourless, and only in rare cases did they give figures in convergent 
light sufficiently good to obtain the sign. It was, however, obtained 
with most of the minerals, but specimens of the rock were also 
crushed, sieved, and separated by methylene iodide. Separation was 
also done, and the specific gravity of the minerals obtained, by means 
of the diffusion column and confirmed by the refractive index, and in 
the case of diopside by the micro-chemical test for magnesium. 
The first specimen examined is chiefly composed of two minerals, 
one of which is of a different refractive index from the other. Both 
are biaxial, and were found to be wollastonite and diopside. Further 
search revealed grains of a negative uniaxial mineral which proved to 
be scapolite. A little magnetite is scattered through the section, 
while anorthite is also present in small quantity. Another section 
from near the above shows crystals of a larger size, and in places 
erystals of calcite have enclosed idiomorphic crystals of other minerals 
in a peecilitic fashion (Pl. V, Fig. 3). These latter include long crystals, 
about three times as long as broad, of zoisite, having a rough cross 
parting, low polarization colours, and a high refractive index. They 
frequently show twinning in plane polarized light. Scapolite with 
a negative uniaxial figure, straight extinction, and moderately bright 
polarization colours is also to be seen with good crystal outlines, as is 
wollastonite with still brighter colours. Magnetite is scattered rather 
thickly throughout the section. 
A third specimen is interesting as it shows a section of the rock 
with the magnetite almost absent from one half and thickly scattered 
over the other half (Pl. V, Fig. 2). All the above-mentioned minerals 
are present in some part of the section, but the grouping is very 
variable. 
