None of the mineralogical problems of the present day are of greater importance than those which deal with the nature and causes of the modifications which crystallised bodies undergo as a consequence of changes in their environment. It is above all things necessary that we should clearly discriminate in every case between the characters of minerals which are original and essential, and those which are accidental and secondary, and which result from the action of various mechanical and chemical forces upon crystals, subsequently to their formation. No fact is more certain than that crystallized minerals, without losing their identity, may have their chemical composition altered within certain (often very wide) limits; and that, as the result of chemical action or mechanical strains, profound changes may take place in all their optical and other physical properties.

Riebeckite was first described by Sauer in 1888. The type-mineral occurs in granite from Socotra collected by Dr. E. Riebeck.

In the same year Harker identified the peculiar blue hornblende occurring in the quartz-porphyry of Mynydd Mawr with the mineral described by Sauer, and in the following year two fresh localities were given for this mineral by Le Verrier and Lacroix. The last mentioned observer has also shown that croeldolite must be referred to the same group of hornblendes.

The object of this communication is to place on record the occurrence of riebeckite in the so-called syenite of Ailsa Craig, the small island which stands at the entrance to the Firth of Clyde.

In the summer of 1890, in the company of Mr. L. W. Fulcher, I again visited Mynydd Mawr, on the west of the Snowdon area, in the hope of obtaining specimens of Riebeckite larger than those on which Mr. Harker and Professor Bonney based their observations in 1888, when they independently pointed out the occurrence of this mineral for the first time in the British Isles. Previous traverses of the dome-shaped mass had shown how uniform in structure and how minutely crystalline the rock of Mynydd Mawr was over all its surface; we accordingly examined the columnar cliffs on the north and west, paying especial attention to the great western hollow, where denudation allows one to stand almost in the centre of the intrusive neck. But even here we were somewhat disappoinied, and our specimens are only slightly coarser in grain than those studied with such striking results by Professor Bonney.

About two years ago Mr. T. A. Herstand of Mainton Springs, Colorado, made a remarkable find of tysonite and bastnäsite at Crystal Park, near Malnten Springs, Colorado. The entire find consisted of one crystalline cleavable mass of tysonlte, covered with bastnäsite, weighing in all fully six kilograms. It was found in a coarsely crystalline granite, and no other deposit has been discovered subsequently, although considerable search has been made by various prospectors. Up to the time of this find all the tysonite discovered in Colorado has not amounted to more than 50 grams.

The tysonite was Principally of a rich cinnamon-brown colour with a perfect basal cleavage, and frequently contained patches from one to three cm. in diameter which were entirely transparent.

The process to be described has been applied to the minerals mentioned below, but I have no doubt that all similarly constituted minerals will also be easily recognised by the method.

I am quite sure that all mineralogists have experienced much annoyance and difficulty in obtaining really satisfactory results with the ordihary blowpipe reactions in regard to some of the compounds of arsenic, antimony, and sulphur with metals, particularly in the separation of the metal or metals from the above-mentioned elements. In order to obtain the metal the assay requires a most careful and tedious roasting on charcoal in the oxidising flame of the blow-pipe, in order to volatilise away the sulphur, arsenic, or antimony. When this roasting is complete the residue (oxide) is mixed with either sodium carbonate, or that salt and cyanide of potas. slum, and fused for a considerable time on charcoal in the reducing flame in order to obtain a bead of the metal.

In this paper the author described his method of constructing crystal models of paper or cardboard by means of plaiting. In the ordinary crystal "nets" (such as, according to Mr. Gotham, were constructed by himself and shown at the Royal Society as early as 1847) the faces of the model are drawn in juxtaposition on cardboard and cut out, some of the lines being only partially cut through ; the net is then folded up so as to form a solid figure, and the faces are united at their edges by gum.

In the models described and exhibited by Mr. Gotham, the faces are so drawn as to be disposed along strips; these strips are united to each other at one end, and can then be folded together by a process resembling plaiting, in which the strips pass alternately under and over each other.