ROCKS. 



[ 662 ] 



ROCKS. 



and tachylytes, microliths and other minute | 

 structures are almost always present. Some 

 of these are mere opaque hairs or threads, 

 called trichites, strangely bent and twisted 

 (PI. 42. fig. 3), and concerning whose 

 mineralogical character nothing definite is 

 known. Others are fine needles, sometimes 

 with lateral growths, which, as in the 

 pitchstone of Corriegills in Airan (PI. 42. 

 fig. 5), resemble, when highly magnified, 

 the fronds of ferns. These spicular bodies 

 or belonites have been referred to augite. 

 In many of the obsidians and pearlites, the 

 microliths are minute elongated prisms or 

 cylinders, and their ends often appear to 

 be rounded. They commonly form dense 

 streams, their longest axes lying in approxi- 

 mately uniform directions, except where 

 larger crystals occur; and at these points 

 the streams are deflected and appear to 

 sweep round the obstacles. In some of the 

 obsidians and vitreous basalt-lavas, the cry- 

 stallites exhibit a more or less complex 

 structure, as in PI. 42. fig. 4, the relation 

 of which can occasionally be traced to per- 

 fectly developed crystals. In the pearlites 

 and in some other glassy rocks, curious 

 little cracks occur, which separate the rock 

 into, minute spheroids (PI. 42. fig. 7). 

 The cracks are seldom continuous all round 

 the form, but thin away in a very cha- 

 racteristic manner. There is no nucleus, 

 and the phenomenon appears to be solely 

 due to shrinkage upon cooling. The streams 

 of microliths which often traverse these 

 rocks, pass without interruption through 

 this pearlitic structure. 



Spherules are sometimes plentifully deve- 

 loped in vitreous rocks, and result in many 

 cases from crystallization around a point or 

 along a line (axiolites) of limited extent. 

 PL 42. fig. 6 shows parts of two spherules, 

 in an obsidian from the Lipari Isles, which 

 have partially coalesced ; and in some speci- 

 mens they may be found united in coo- 

 tinuous'strings. 



In the felspars and other minerals occur- 

 ring in certain lavas, enclosures of vitreous 

 matter are sometimes visible. These are 

 small portions of the glassy magma which 

 have been entrapped by the crystals during 

 their formation. PI. 42. fig. 2* shows glass 

 lacunae of this kind enclosed within a fel- 

 spar crystal in one of the lavas of Etna, 



The crystalline eruptive rocks consist, p.s 

 already stated, of crystals or crystalline 

 grains of various minerals, and it is neces- 

 sary to study their optical characters in 



order to determine them with any preci- 

 sion. Before entering, however, upon this 

 method of investigation the student should 

 become familiar with ordinary hand speci- 

 mens of the chief rock-forming minerals, 

 and should acquire some knowledge of the 

 simple tests, both physical and chemical, 

 which distinguish the different species. In 

 the absence of such elementary knowledge, 

 deductions based solely upon microscopic 

 observations are sure to be untrustworthy, 

 if not worthless. The text-books from 

 which this rudimentary instruction may be 

 derived are too numerous to need mention. 

 For the study of the optical properties of 

 minerals the student may advantageously 

 consult the works of Dana, Zirkel, Rosen- 

 busch, von Lasaulx, Fouque", Le"vy, and 

 other recent contributors to microscopic 

 petrography. 



In a mere sketch of the subject, such as 

 this must necessarily be, it is impossible to 

 do more than allude to the microscopic 

 characters of a few of the most typical 

 rocks ; and in doing so, we may begin with 

 a thin slice of granite (PI. 42. fig. 9). The 

 student doubtless knows beforehand that 

 he is dealing with a rock composed essen- 

 tially, of felspar, mica, and quartz. But 

 there may be more than one species of fel- 

 spar present, and the mica may also be of 

 various kinds. To determine the character 

 of the felspars, he must study their angles 

 of extinction between accurately crossed 

 NicoPs prisms j and to do this, it is neces- 

 sary to employ a microscope with a pro- 

 perly divided, rotating stage, which can be 

 centred correctly for any object-glass which 

 may be used, so that a minute speck in the 

 preparation can be made to revolve imme- 

 diately beneath the point of intersection of 

 two crossed spider-lines set within the eye- 

 piece. He can, in most cases, ascertain 

 whether the felspars crystallize in the 

 monoclinic or in the triclinic system by ob- 

 serving the character of the twinning in 

 polarized light. The monoclinic felspars 

 are usually twinned on what is known as 

 the Carlsbad type, and show only two dif- 

 ferently coloured halves or lamellae ; while 

 the triclinic felspars are striated by nume- 

 rous twin-lamellse, which polarize in dif- 

 ferent alternatiug colours; unless, of course, 

 in either case, the plane of section coincides 

 with the plane of composition of the twin- 

 lamellae, i. e. the direction in which the 

 plates appear to be stuck together. In the 

 monoclinic felspars, the maximum extinc- 



