218 W. M. Hutchings—Ottrelite in North Cornwall. 
definite boundaries, and indications of crystal faces at the edges. 
The average diameter of the flakes is about ;45 inch, and the 
thickness of the opaque ones ranges between 33455 and yoyo inch. 
Like the ottrelite, the ilmenite lies in all directions with regard 
to the cleavage of the slate, but a considerable majority of the flakes 
are approximately parallel to it. 
A large part of this ilmenite is reticulated, in varying degree, with 
sagenitic rutile. Among the thinner transparent flakes all degrees 
may be seen, from such as show a wholly unbroken brown colour, 
to others which are so finely meshed with delicate interlacing lines 
of rutile, that only a high power suffices to display the structure ; 
and there are, finally, bits of sagenite in the network of which no 
trace of ilmenite is seen. Thicker plates of ilmenite also often show 
a. development of sagenite, while many show only a line here and 
there. Such of them as are still thin enough to be at all translucent 
frequently show indications, in strong light and under high power, 
that a little thinning would expose complete reticulations. 
Rénard has devoted considerable study to similar occurrences of 
ilmenite in rocks of the Ardennes. In a paper published in 1884 
(Recherches sur la Composition et la Structure des Phyllades 
Ardennais, Bulletin du Musée Royal de Vhistoire naturelle de 
Belgique, tome iii. pp. 231-268) he described in detail, and figures 
the “phyllade 4 ilménite des Forges de Ja Commune.” The 
occurrence in this case is clearly of the same nature as in the 
Cornish slate, this being borne out by sections of the Belgian rock 
which I have examined; the difference being that in the latter the 
flakes are a good deal larger, while the amount present is very much 
less, as is also the proportion of it which is transparent or in which 
sagenite is developed. Rénard was at great pains to isolate some of 
the mineral and prove chemically that it is ilmenite. In the Cornish 
rock, in face of the large amount of transparent brown flakes, such 
chemical proof would scarcely be called for; but it is all the better 
that Rénard carried it out, inasmuch as it does not seem practicable 
to isolate the much smaller flakes and tablets of the Tintagel slate. 
No better material could be desired than these North Cornish 
phyllites for the study of these highly interesting microscopic 
sagenites and their combinations with ilmenite; and in view of the 
large amount of very thin, thoroughly transparent flakes so com- 
bined, I have been in hopes that by careful study of them some 
light might be thrown on the question as to how the combination 
ought to be regarded ; whether the sagenite is a secondary product 
resulting from the decomposition of the ilmenite; whether the two 
have been originally formed together as now seen; or whether, in 
any cases, the intervening ilmenite has resulted from an alteration 
of the rutile, the occurrence of alterations of rutile into ilmenite 
having been demonstrated by V. Lasaulx. 
The first of these alternatives is that which most readily suggests 
itself on first examination of the minerals; and it might seem to 
derive some support from Cathrein’s observation that ilmenite dis- 
solved in acid frequently leaves a residue of extremely minute 
