178 



THE POPULAR EDUCATOR. 



rocks, they are never found associated with recent accumula- 

 tions, but they have a far wider range, from the Silurian forma- 

 tion up to the Tertiary, both inclusive. But the division between 

 volcanic and trap rocks is more one of position than distinct 

 mineralogical character. This was expressed by Sir James Hall 

 when he said, " I am confident there is not a lava of Mount Etna 

 to which a counterpart may not be produced from the whin- 

 stones of Scotland," which are traps. 



The traps are decidedly of igneous origin ; we find them 

 disrupting strata, and filling up the fissures, while the rock in 

 the neighbourhood exhibits marks of induration, and other 

 alterations by heat. The trap rocks bear a general division 

 into feldspathic and augitic traps. 



FELDSPATHIC TRAPS. 



In these rocks feldspar predominates. Felstone, or compact 

 feldspar, is almost a pure mass of this mineral ; it is a compact 

 flinty rock. Its colour is either grey, of various shades, or a 

 greenish white ; this latter kind is often translucent at the 

 edges. 



Pitchstone is a variety of felstone having a resinous lustre, 

 hence its name. 



Felstone porphyry has a basis of compact feldspar, with large 

 crystals of the same mineral disseminated through the mass. 

 The rock to which the term porphyry was originally, and is still 

 applied, was of this very nature, the basis being of a dark 

 brick-red colour, and the crystals white, or more generally of a 

 flesh-colour. The appearance of such a rock is indicated by 

 Fig. 23. 



AUGITIC KOCKS. 



In these the mineral augite is the chief ingredient. Augite 

 and hornblende are considered by many mineralogists as 

 synonymous terms. 



Basalt, which we have briefly described, is a prominent trap 

 rock, and the basalts of the north of Ireland are of this augitic 

 class. 



Greenstone is applied to a large class of trap rocks. Their 

 colour is a dark green, owing to the presence of hornblende in 

 great quantity. They are less compact than the basalts, .and 

 exhibit distinct crystals of their several ingredients. They are 

 known in Scotland as v-hinstones. 



When the composition of greenstone is mainly feldspar and 

 augite, then it is called dolerite. 



Serpentine, so much used to make ornamental vases, pillars, 

 and mantelpieces, belongs to this class of rocks, though some 

 geologists class it with the granites. 



The enclosed minerals we shall treat of in another place. 



THE GRANITIC KOCKS. 



These rocks superabound in silica. The volcanic and trap 

 rocks are chiefly classed according to the quantity of silica they 

 contain, but it is always compounded with some base, such as 

 potash, soda, alumina, or magnesia. But in the granites there 

 was not sufficient of those bases for the silica to take tip ; hence 

 it separated out, and can be distinguished in the rock in grains 

 of transparent quartz. 



True granite is readily recognised ; as the derivation of the 

 word indicates ygranum, a grain), it is constituted of grains, 

 very perceptible. 



In a typical specimen those grains are 1. Quartz. 2. Feld- 

 spar. 3. Mica. 



1. The quartz is commonly colourless, very rarely brown ; it 

 is easily recognised, being not unlike a piece of glass. 



2. The feldspar may be ortlioclase, or potash feldspar, which 

 is generally flesh-coloured ; or albite, or soda feldspar, which is 

 usually dead white. These distinctive minerals are, however, only 

 the heads of two classes, which contain a great variety, and 

 which are found frequently mixing. 



3. Mica is sometimes in small colourless plates, which give 

 to the granite of which the houses in Dublin are built its 

 spangled appearance ; more generally it is black, and between 

 these extremes is found in every shade of grey. 



To gather the relative proportions of these constituents, wo 

 give this analysis of Wicklow granite by Professor Haughton : 



Feldspar 

 Quartz 

 Mica . 



61-18 

 24-98 

 13-37 



99-53 



The grainc of these minerals exhibit no signs of the action of 

 water. They are not water-worn, neither is there any appear- 

 ance of stratification. The nearest approach to it is in the case 

 of graphic granite, the vertical section of which is drawn in Fig. 

 24, and the horizontal section in Fig. 25. 



It will be seen that the black mica in the horizontal section 

 has some resemblance to Hebrew characters, hence its name 

 graphic, 



Syenite, named from Syene, an Egyptian city, has four con- 

 stituents ; the mica is in a very small quantity, and has been 

 almost wholly replaced by hornblende. 



Protogine, or "first-formed" so called from an erroneous 

 supposition that it was formed before granite has talc in its 

 composition instead of mica. 



Granitic rocks have evidently never cooled at the surface, for 

 we never find any ashes, or anything corresponding to pumice : 

 all is hard and compact. That they were once in a molten con- 

 dition seems an inevitable conclusion, when we inspect' such 

 diagrams of granitic veins as those taken from Lyell's 

 " Elements," and drawn in Figs. 26 and 27. 



These veins are never found traversing rocks higher than the 

 cretaceous, and instances are found in Norway where the 

 granitic vein penetrates the strata without dislodging it or 

 altering its dip ; so that either the fissure must have been made 

 by the shrinkage of the rock, on account of the proximity of 

 heat, or else that after the upheaval took place the strata 

 .returned to its normal position. 



LESSONS IN GREEK. XXVIII. 



THE student will find an advantage in comparing together tli9 

 three voices. The relation of their leading parts may be seen 

 in the following 



CONSPECTUS OF THE THREE VOICES. 

 INDICATIVE MOOD, FIRST PERSON SINGULAR. 



Active Voice. Middle Voice. Passive Voice. 

 Present. Au-co, AU-O-/UCU. 



Imperfect. e-Au-op, f-\v-ofj.riv, 



Future. Au-tr-co, Ati-cro/tai, Xv-Ort-ff-ofnat. 



First Aorist. e-Av-cr-a, e-Au-crajwjv, 



First Perfect. Ae-Au-/c-a, 

 First Plup. e-Ae-Au-K-eij', 

 Second Perf. ire-^rjj'-a, P.F. Ae-Au-er-o/uai. 

 Second Plup. e-Tre-^rji/-*!?, 

 Second Aor. t-\nr-ov, f-\nr-ofj.r]i>, 



Second Fut. 



GENERAL CONSPECTUS OF THE GREEK VERB. 



ACTIVE VOICE. . 

 Indie. Subj. Opt. Imp. Infin. Part. 



f-\v-Qr\v. 

 Ae-Au-/xa4. 



Pros. Aua>, Avco, 



Imp. t\voi>. 



Fut. \va-ta, 



1 Aor. sAwra, \vo-ca, 



1 Perf. AeAu/co, AeAu/cco, 



1 Plup. t\f\VKfiv. 



2 Perf. ire</>T7//a, TTf<f>r)v<a, 

 2 Plup. eire^Tjj/eiy. 



2 Aor. eAnrov, \tirta, 



\v<roim, 



fat, 



Ai/, \vftv, \va>v. 



Kvffeiv, \vffcav. 



\vffov, \vffai, \vaas. 



AeAu/ce, AfAuwe- AsAu/cws. 

 vai, 



vat, 

 Aiirew, 



\nruv. 



AlTTUljUl, Al7T6, 

 MIDDLE VOICE. 



Pres. Auo/xai, Aucojuai, \voifirjv, Ai/ou, \vtff6ai, Auo/uvos. 



Imp. (\vofj.rji>. 



Fut. Aixrojuai, \vffoifjii)v, \vfffff6ai, \vffo/j.evos 



1 Aor. f\v<rafj.r/i',\va'ei>fj.a.i, Atxrcu/iTji', Aucroi, Au<ra<r0oi, Avcra/xsws 



2 Aor. f\nrofj.riv,\nr(afjLai, AJTTOIJUTJV, AITTOU, \nrtff9at, AJTTO/ASVOS 



PASSIVE VOICE. 



1 Aor. eAuOrji', Auflco, \v8tiriv, \v9rjTi, \v9nva,t, \v6eis. 



1 Fut. AuOrjcro- AuflTjcrot- Au0rjcr6(r- \v6r)ffo- 



Perf. AeAu^tai, AeAu/ue- \e\vfif- \t\vffo, \f\va6at, \t\v/jLfvos 



Plup. \f\VfJ.1JI'. VOS, -CO, VOS,-fl1]V t 



P. Fut. \e\ucrofj.ai, \f\vffot- \f\vfftff- AeAucro- 



/UTJf, 8oi, MfOJ. 



2 Aor. frpiftr,v, rpifiw, Tpt&eirii>, rptflr)9i, Tpiftrjvai, rpifleis. 

 2 Fut. Tpi^rjcro- TptfirjffOf Tpiflri<r- 



(Mi, ^TJff effdat, 



