W. M. Hutchings—Ottrelite in North Cornwall. Pall 
Examined between crossed Nicols a large proportion of the lath- 
shaped cross-sections are seen to be twinned, but this twinning does 
not seem to be as pronounced as in some of the other occurrences of 
the mineral, there being mostly only two lamelle even in the 
thicker bits, and many flakes being quite untwinned. 
Many of these sections extinguish quite parallel to their length, 
but the greater number show an angle, which reaches as high as 
20 degrees and over in some cases, though about 12 degrees is a 
more usual figure. 
The examination of selected, sufficiently thick, and untwinned 
fragments from the isolated material in convergent polarized light 
shows the emergence of a positive bisectrix on the principal 
cleavage-face of the mineral. The angle of optic axes is rather 
large. The optic-axial plain bisects the obtuse angle formed by the 
two best-developed secondary cleavages. 
As usual, the ottrelite is rich in inclosures, but is very much less 
crowded than in most other occurrences. The principal mineral 
inclosed: is rutile, in perfectly formed crystals for the most part, 
many being twinned both as “knees” and “hearts.” The largest 
crystals measure sayy inch, and they pass from this into the merest 
specks through all intermediate sizes. Other included matter is 
very small in amount, and mostly quite indeterminable in nature, 
with the exception of white mica which is frequent, and which is in 
many cases seen to occupy cracks and seams in the ottrelite. Many 
bits are also full of cavities of all shapes and sizes. 
Gosselet states that he was able to observe, around the individuals 
of ottrelite, zones which were very much less rich in microlites than 
‘the rest of the rock. This is not the case in the Tintagel slate. 
The whole rock is very rich in minute rutile crystals; but though 
the ottrelite contains them in far greater abundance than does the 
sericite, there is no sign that it has impoverished the space just 
around it. 
No quantitative analysis has been made of the isolated material, 
but it has been tested as to its resistance to acids, and has been 
qualitatively analyzed, the results of these examinations confirming 
the optical and other evidence that the mineral is ottrelite. 
Specimens of the rock have been submitted to authorities in this 
country and on the continent, who also recognize the mineral as 
ottrelite. 
No doubt it will be found in other similar slates when search is 
made, as well as at Tintagel. 
The ilmenite of this rock is well worthy of remark. It is 
present in considerable amount in nearly all the phyllites examined, 
but is perhaps more abundant in this ottrelite-slate than in any 
other. It is pretty evenly diffused in little flakes and tablets. 
Some of it is in the form of the thinnest “‘micaceous ilmenite,” 
perfectly transparent, with rich brown colour, and there is every 
gradation of thickness and translucency, up to tablets which are 
perfectly opaque. The thinner and medium-thick flakes are all 
more or less ragged, but many of the thicker tablets show more 
