EOCKS. 



[ GC4 ] 



EOCKS. 



a rule, no dichroism. Augite is a common 

 constituent of Basalt and other rocks of a 

 basic cliaracter. Fig. 18 shows a section of 

 a crystal of augite cut transversely to the 

 principal axis. Li fig. 14 are shown the 

 very peculiar forms of hornblende, which 

 occur in some phonolites, and which from 

 their frayed-out ends can sometimes hardly 

 be recognized as crystals. In fig. 15 some 

 more hornblende is shown, occurring in a 

 dioi ite ; the character of the twinning in the 

 triclinic felspar is shown in this drawing. 

 It is in the nature of the felspar that diorite 

 differs from syenite. The original syenite 

 of Pliny, from Syene in Egypt, contains 

 quartz. Many petrologists, however, now 

 exclude from syenite those rocks in which 

 quartz is present. The quartzless syenites 

 are closely related to minette (PI. 42. kg. 11) , 

 in which magnesian mica takes the place of 

 hornblende. The crystals of mica constitute 

 the chief features of the rock. The matrix 

 in which they lie is a finely crystalline mix- 

 ture of felspar and mica. In the drawing, 

 the central crystal of mica is cut trans- 

 versely to the basal plane : the line stri;e 

 representing the direction of cleavage or the 

 characteristic platy structure of mica. A 

 crystal of magnetite is also shown in this 

 drawing. It is a section through an octo- 

 hedron. Among the minerals which crys- 

 tallize in the cubic system, Haiiyne and 

 Noseaii are often met with in volcanic rocks. 

 In PI. 42. fig. 14 some crystals of Noseau 

 are shown ; like all cubic minerals, they 

 exert no influence upon polarized light. 

 Haiiyne and Noseau exhibit structiu'al pecu- 

 liarities when examined under high powers, 

 which cannot be described or figured in this 

 short article. Fig. 16 represents a crystal 

 of Olivine in a Saxon basalt. Olivine is 

 a common constituent of basalt, and some 

 modern petrographers exclude from basalt 

 any rock from which olivine is absent : thus 

 many of the rocks which in England have 

 always been regarded as basalts, would 

 under this new arrangement be classed with 

 the andesites. Olivine being a rhombic 

 mineral, has its axes of elasticity coincident 

 with its crystallographic axes. Fig. 17 

 represents a section of basalt from the 

 Giant's Causeway, as seen by polarized 

 light under an amphfication of 77 diameters. 

 The black patch in this drawang indicates 

 magnetite or titaniferuus iron, both of which 

 minerals are commonly more or less plen- 

 tiful constituents of basalt. PI. 42. tig. 15 

 shows a transverse section of a crystal of 



Nepheline partly filled with dusty matter. 

 Nepheline is sometimes met with in bascilts ; 

 this drawing is made from a phouolite. The 

 phonolites are rocks which essentially con- 

 tain nepheline. The green crystals in the 

 drawing are microliths of hornblende. 



The rocks Diabase and Gabbro may be 

 regarded as varieties or special conditions 

 of basalt. The latter rock contains diallage 

 instead of augite ; but these two mineral 

 species ai-e now regarded by some mineralo- 

 gists as identical. 



The rocks termed Norite and Hj'persthe- 

 nite, contain the rhombic forms of pyroxene, 

 hyjiersthene, and enstatite. Fig. lu is 

 drawn from a section of trachyte and shows 

 crystals of sanidine and oligoclase,the former 

 a monoclinic, the latter a triclinic felspai-. 

 In the centre is a crystal of sphene or 

 titanite, which although not a very common 

 rock-forming mineral, nevertheless occurs 

 both in plutonic and volcanic rocks. 



We now come to rocks of a diti'erent class 

 to those just described. These are kuo-svn 

 as jnetamorpJuc ; and they are regarded by 

 some as the result of the alteration of sedi- 

 mentary rocks by high temperatures existing 

 at great depths beneath the surface of the 

 earth, or by the contact or proximitj' of 

 eruptive rocks. Others regard them in 

 many cases as the residt of crystallization 

 from solution, and not by fusion, set up in 

 sedimentary rocks subsequent to their depo- 

 sition. Whatever may have been the pro- 

 cesses by wdiich they have arrived at their 

 present condition, we find that they consist 

 of minerals which, in the main, are identical 

 with those which are met with in eruptive 

 rocks : thus, gneiss for example, is ideutic-al 

 with granite so far as its mineralogical con- 

 stitution is concerned ; but the crystals and 

 crystalline grains which compose the rock, 

 are arranged in more or less definite layers 

 of different mineral character, an arrange- 

 ment termed foliation. Mica-schist (PI. 42. 

 fig. 20) is another example of a metamor- 

 phic rock, and consists essentially of rudely 

 alternating layers or films of mica and 

 quartz. Chlorite-, talc-, schorl-, and horn- 

 blende-schists are rocks of similar structure, 

 and contain respectively the minerals which 

 their, names imply, together with quartz, 

 and at times felspars, garnets, &c., while 

 staurolite, chiastolite, kc. are met with in 

 certain argillaceous rocks which have also 

 been subjected to alteration. Quartzite 

 again is an .altered sandstone, and the bed- 

 ded hjillefiiutas of Scandinavia (PI. 42. 



