ROC lis. 



[ 665 ] 



ROCKS. 



fig. 12) are usually regarded as altered 

 fel;<patliic .saiidstoues, which are identical in 

 miuural cous^tilu'iou aud structure with 

 the rocks liuowu as t'elstoue, wliich coosist 

 of a luicro-crystaUine admixture of felspar 

 and quartz, and in which the boundaries of 

 the component grains are uot always clearly 

 defined under the microscope. This hazy 

 ill-detined granular structure is spoken of 

 as a crypto-crystalliue one ; while there is 

 ako another condition of matter having the 

 same chemical composition in which merely a 

 nebidous, granidar, or faintly-marked tibrous 

 structm-e distinguishes it from vitreous sub- 

 stances, its optical character being similar 

 to that of glass. This is known as micro- 

 felsitic matter. 



The four drawings which constitute the 

 lower row in Plate 42 represent sedimentary 

 rocks, or those which have resulted from 

 the disintegration of preexisting laud, the 

 degi-aded ruaterials having been transported 

 by rivers aud deposited in the sea or in lakes. 

 Sandstones, shales and slates, and lime- 

 stones, may be regarded as the principal 

 t^pes of these sedimentary deposits. PI. 42. 

 fig. 21 represents a section of millstone grit 

 as seen under polarized light. The rock 

 consists essentially of fragments of quartz 

 with some fragments of felspar, bound to- 

 gether by a ferruginous cement. The frag- 

 ments have probably resulted from the 

 disintegi-ation of granitic rocks, the mica, 

 owing to its form rather than its specific 

 gravity, having been held longer in suspen- 

 sion, and transported to greater distances in 

 the old seas in which these materials were 

 deposited. PI. 42. fig. 22 represents a piece 

 of Cambrian slate from the Penrhyn quar- 

 ries in N. AVales. The appearance is that 

 presented between crossed Nicols. The 

 general elongation of the constituent particles 

 in the direction of cleavage, is the result of 

 pressure, which is further demonstrated by 

 the frequent distortion of the fossils met 

 with in these old deposits. Microliths are 

 also present whicb have been developed 

 subsequently to the deposition of the rock. 

 Similar microliths in other Palaeozoic slates 

 have been referred by Kalkowsky to anda- 

 lu,~ite, a silicate of alumina. PL 42. fig. 2-3 

 shows the small spheroidal grains which 

 occur in the Great Oolite of Lincolnshire. 

 They are concentric deposits of carbonate of 

 lime, which often, as in the largest spheroid 

 in the figure, have a foraminifer as a 

 nucleus ; in some cases the accretions have 

 taken place aroui.d a grain of sand. The 



interstitial matter in these rocks is usually 

 crystalline carbonate of lime. PI. 42. 

 fi;,'. 24 is an example of a crystalline, sac- 

 charoid limestone, statuary marble. It 

 consists wholly of crystalline grains of 

 calcite. The twinning of the separate crys- 

 tals, parallel to a rhombohedral face, i3 

 reju-esented as it is seen by polarized light. 

 It is impossible in this article to cite 

 further examples. Volcanic ejectamenta, 

 which both in their loose and consolidated 

 states, oiler problems of great interest to the 

 geologist, must pass unnoticed ; but, before 

 closing this article, it may be worth while 

 to make a few remarks on the relative 

 merits of thick and thin microscopic sections 

 of rocks. In the first place thick sections 

 are easier to prepare ; but in most cases 

 they are of little or no use to the student, 

 since the optical properties of the different 

 minerals cannot as a rule be properly 

 studied from such feebly translucent pre- 

 parations. The rotatory polarization of 

 quartz cau of course only be observed in 

 thick sections taken at right angles to the 

 optical axis ; and dichroism, when barely 

 perceptible in certain thin slices, becomes 

 well marked in thicker ones; but apart 

 from these advantages which thick sections 

 possess, they have others. A thick un- 

 mounted section can be submitted to the 

 action of chemical reagents, and in many 

 cases may be subsequently washed, ground 

 thinner, and finally mounted, while much 

 useful information may be thus gained. 

 Little objection can also be raised to thick 

 sections of vitreous rocks, when they happen 

 to be very transparent, since the crystals 

 which theycontain may then be studied in 

 their integrity ; whereas in thinner sections 

 we can merely examine slices of them. 

 Furthermore, in the preparation of very thin 

 sections there is always the danger either 

 of stripping out the larger crystals, or of 

 ultimately wasting labour by grinding away 

 the whole of tlie preparation. Thick sec- 

 tions are best examined by reflected light, 

 when many important points are often dis- 

 cerned,_ which would never be noticed if 

 transmitted fight only were employed. As 

 a rule, however, it is scarcely possible to 

 prepare too thin a_ section of a rock, nor to 

 over-estimate the value of the thinnest pre- 

 parations which can be procured; since a 

 few finishing touches may often permit the 

 recognition of structure which would other- 

 wise pass unnoticed. Most of the lapi- 

 daries in this country spoil their sections 



