In the following short paper I propose to describe what I have lately done in improving the method of studying the optical characters of the minerals which I treated at greater length in my address last year at annual meeting at Plymouth.

It is a curious example of how a method may be invented, and then lost sight of, that the determination of the index of refraction in the way there described was proposed by a French savant upwards of a hundred years ago. I have not yet consulted the original publication, but I very strongly suspect that the proposal was more theoretical than practical, and that with the instruments then at disposal the results were found to be so inexact that the whole system became obsolete and practically forgotten.

This lies to the south of Unst ; it is interesting as forming the connecting link which unites the rock masses of Fetlar with those of Unst.

Its western extremity is composed of diallagic rocks, containing crystals of that mineral (Hb) very inferior to those of Balta : its middle portion consists of various rocks of the "chloritic series" ; towards the east there is seen a bed of graphitic slate (H) quite similar to, and doubtless the continuation of one which appears near Trista V oe in Fetlar.

My original account of the mineral which I called Mordenite, from having first obtained it in the trap of the bay of Fundy, at Morden, King's Co., N.S., is to be found in the Journal of the Chemical Society, t864. A quotation is made from this to show the peculiarity of the deposit I am about to describe, and a few additions are given respecting opinions expressed on the species, and on its supposed occurrence elsewhere, before taking up the account of the recent observations to which I desire to draw attention.