As long ago as 1816, Brewster showed that many crystals have a lamellar structure developed in them by pressure, and his observations became the starting point of a long series of very valuable researches. Reuseh proposed that the planes along which such structures are developed by mechanical means should be called "Gleitflächen" (gliding-planes), and he, Pfaff, and other mineralogists have investigated a number of curious cases of the kind. Baumhauer in 1879 showed that the lamellar structure could be brought into existence, in the ease of calcite, by simply inserting s knife-blade in a definite direction into cleavage fragments of the mineral, and his observations were confirmed in the following year by Brezina.

Before describing some features of special interest in the microporphyritic quartz crystals of an Indian felsite, it may be as well to say a few words, by way of preface, regarding the rock itself.

About 85 miles to the north-west of the town of Delhi, in the peninsula of India, a hill rises abruptly from the plain to the height of 630 feet. It is one of a series of similar elevations separated from each other by some miles of sandy soil which strike the traveller with their resemblance to a group of islands in a tempestuous sea—an idea greatly favoured by the long swelling waves into which the sand is thrown by the strong westerly winds which prevail in that region during the greater part of the year.

I propose to add one more account to those which have already been published of this mineral. It is described as a rhombie mineral in Dana's Mineralogy and other standard works. Latterly it has been described by Reusch as an anorthic mineral, from National Belie Mine, Red Mountain, Colorado, in the Jahrbuch für Mineralogie, 1887, Bd. II. Now I describe it as monoclinic, principally from a study of that variety of it which occurs in Anglesey, of which an account was published by Dr. Percy in 1875 in his Metallurgy, Vol. "Fuel."

The mechanical inclusion of foreign substances in crystals during their formation is of such common occurrence that it might appear superfluous to record further examples, were it not that the present ease is a somewhat unique one. All mineralogists are acquainted with the inclusion of sand and clay in selenite, celestite, and calcite, of which the Fontainebleau limestone is a striking example. Delesse found the crystals of calcite from that locality to contain 57 per cent. of sand, and to have a sp. gr. of 2·84, whilst the sp. gr. of pure calcite is given by Beudant as 2·721 to 2·723, which demonstrates that the inclusion of the heavier quartz grains raises the sp. gr. of the impure crystals.

The part played by hornblende (i.e. the monoclinie amphiboles) in the constitution of rocks, and the relation of that mineral to angite (including all the monoclinlc pyroxenes), are subjects which, in recent years, have occupied much attention. It has been recognised that the early notions of paragenesis cannot be applied, without many reservations, to rock-forming minerals. Some petrologists have gone so far as to suppose that augite and hornblende "are two different crystallographic forms of essentially the same molecule, of which the former is most stable at high, the latter at ordinary temperatures." If this view receives any extensive confirmation, the well-known conversion of angite into hornblende must be regarded as a purely paramorphic change due to altered conditions.