Short Notes and Queries. 



77 



want to know if this is the method in which our friend obtains his 

 delicacies ; but whether or not, I venture to think the fact is worth 

 knowing. I shall be particular in watching the species further. Quite a 

 couple of hours were devoted to Mr. Beetle, when I was obliged to run to 

 catch our departing steamer. I have enclosed specimens, also a small 

 co3kroach, the pest of the vessel, and a species of Zygoena (found in 

 swarms at Kondebosch, near Cape Town), and I want you to get me the 

 names of all three, as they will be useful to me. — S. D. Bairistow, Steam- 

 ship Balmoral Castle, off Cape Town, September 30th. (Extract from 

 letter to Mr. Porritt.) 



Rocks Microscopically Examined. — On the 22nd of November, Mr. 

 Thomas Tate, F.G.S., delivered an address before the Leeds Geological 

 Association, in the Yorkshire College (chairman. Professor Green, 

 M.A., F.G.S. ,j his subject being — " The Determination of the Mineral 

 Constituents of Eruptive Pocks Microscopically." The optical behaviour 

 of mineral crystals in thin slices beneath the microscope may be observed 

 in four ways. By transmitted light they appear either colourless, 

 coloured, or opaque. They may next be examined by the polariser alone, 

 when some will shew alterations of the colour from light to dark shades 

 as the polariser is rotated (dichroism), while others will remain unaffected 

 (non-dichroic). If the analyser be now added, those minerals which 

 depolarise light will give more or less brilliant chromatic effects when the 

 polariser is rotated (anisotropic) ; while other crystals will give no colour 

 changes, but will merely darken between crossed prisms (isotropic). The 

 glassy forms, for example (obsidian, tachylite, . &c, ), are isotropic. The 

 commonest colourless sections are those of quartz, felspars, leucite, nephe- 

 line, enstatite, olivine, apatite, all of which are usually anistropic save 

 leucite, which grows dark between crossed Nicols, and apatite, which 

 remains bright in all positions of the prisms. Enstatite is also dichroic. 

 Muscovite, brotite, hornblende, hypersthene, augite, and diallage are 

 coloured — anistropic and dichroic — except the last two, which are non- 

 dichroic. Magnetite and iron-pyrites are opaque ; but viewed by reflected 

 light, the former is of a leaden and the latter of a brassy hue. The 

 advantages which this most recent method of research offers are very 

 decided. By it the investigator is enabled to determine most accurately 

 the character and relative amounts of the constituent minerals present in 

 compact micro-crystalline rocks, whose constitution has been hitherto 

 little more than guessed. The large crystals in rocks having a more 

 granite structure, are by this method often found to be crowded with 

 enclosures of foreign minerals entangled during the process of con- 

 solidation, the presence of which, in hand specimens, would be unsuspected, 

 but which must vitiate their chemical analysis ; and this fact, revealed by 

 the microscope, doubtless accounts for the wide variation in the published 

 percentage of the chemical elementary constituents. Further, by a care- 

 ful study of the glassy forms, with their fluxion structure of streaming 

 microliths or embryo crystals surrounding the entangled crystals, and the 



