HARDWICKE'S SCIENCE-GOSSIP. 



101 



Fig. 



diameter, and adapted to carry a porcelain or copper 

 hot-water bath, or a small brass plate. The former 

 is illustrated in section at H, and the latter at I, and 

 their use to the microscopist in the practice of pre- 

 paring and mounting objects is not easily overrated. 

 The larger ring carries the ordinary reading-shade, 

 and both are concentric with the burner D. Three 

 carriers of the form J are attached radially 

 to the burner D, and upon them the porcelain 

 white cloud-shade K rests, while a blue spot 

 upon the lamp-glass of D corrects the yellow 

 rays (less in number in the Silber burner at 

 any time than in ordinary petroline flames), 

 and renders this, to my mind, the most 

 powerful and most complete microscope lamp 

 known. 



I shall have the pleasure to introduce the 

 first one manufactured to the notice of the 

 members of the Royal Microscopical Society 

 at their next meeting, and immediately after- 

 wards they will be supplied by the Silber 

 Light Company (Limited) and their agents ; 

 but I should perhaps at once say that I have 

 no interest whatever in their manufacture, 

 beyond a desire that the " Sear Lamp " may 

 prove a useful and convenient accessory to 

 the use of my favourite instrument, the 

 microscope. W. Lane Sear, 



Hon. Lib. Margate Micro. Club. 



A CHAPTER IN THE HISTORY OE ROCK 

 STRUCTURE. 



By the Rev. J. Magens Mello, M.A., F.G.S., &c. 



President of the Derbyshire Microscopical Society. 



THE study of rock structure is one of great in- 

 terest to the geologist, and not only does it 

 teach him the various materials of which any parti- 

 cular rock is built up, but it will often lead him to 

 the knowledge of wonderful facts relating to its 

 origin and past history, and will enable him to trace 

 some of the many changes to which it may have 

 been subjected during the lapse of time. 



I propose to illustrate this by taking some familiar 

 specimen and showing the ways in which we may 

 investigate its nature and history. 



Suppose we take a piece of granite and see what 

 we may learn about it. There are few persons but 

 are acquainted with this rock in some one or more 

 of the forms in which it is found. Our public build- 

 ings often present us with splendid illustrations of 

 granite, sometimes roughly hewn, as it has come 

 from the quarry ; in other cases highly polished. 

 We have seen the fine grey stones from Aberdeen, 

 or the beautiful red ones from Peterhead and else- 

 where. Now when we begin to examine a piece of 

 one of these granites, we see at once that it is not a 



homogeneous stone — such, for instance, a3 is a bit 

 of flint — but that it is built up of various dissimilar- 

 looking materials ; and we may notice, moreover, 

 that one or more of those materials is crystallinet 

 that it is shaped in some regular geometrical form. 

 We shall probably be struck with certain whitish or 

 flesh-coloured crystals, more conspicuously promi- 



49. Section of granite from Cornwall (polarized), magnified 

 26 diam. 



nent than the other substances of which the speci- 

 men is composed. With some care we may be able 

 to make out in part the form of these crystals, and 

 perhaps to measure one or more of their angles 5 



Fig. 50. Orthoclase Felspar. 



Fig. 51. Plagioclase Felspar. 



Fig. 52. Mica (Biotite). 



then, too, we shall notice that these crystals are 

 apparently embedded in a more glassy-looking sub- 

 stance of a clear or greyish colour, and here and 

 there we shall observe some bright spangles of a 

 thin flaky mineral. We shall thus have seen the 

 three principal minerals of which typical granite 



