File:The Quarterly journal of the Geological Society of London (13937219641).jpg

COLLECTED

 IN 
 MADAGASCAR 
 BY 
 REV. 
 R. 
 BARON". 
 349 
 

The

 doleritic 
 type 
 is 
 also 
 represented. 
 Such 
 a 
 rock 
 occurs, 
 for 
 

instance,

 at 
 Mojanga 
 on 
 the 
 north-west 
 coast 
 of 
 Madagascar. 
 In 
 

this

 rock 
 there 
 is 
 a 
 marked 
 ophitic 
 structure, 
 large 
 allotriomorphic 
 

grains

 of 
 augite 
 being 
 penetrated 
 by 
 prisms 
 and 
 microlites 
 of 
 felspar 
 ; 
 

the

 rock 
 is 
 nearly 
 noncrystalline, 
 glassy 
 matter 
 being 
 present 
 only 
 

in

 thin 
 films 
 between 
 such 
 felspar 
 needles 
 as 
 are 
 not 
 enclosed 
 in 
 

the

 augite. 
 

A

 very 
 curious 
 rock, 
 which, 
 though 
 diverging 
 widely 
 from 
 the 
 

normal

 structure 
 of 
 the 
 olivine 
 basalts, 
 may 
 here 
 be 
 conveniently 
 

described,

 occurs 
 three 
 or 
 four 
 miles 
 east 
 of 
 the 
 village 
 of 
 Eeno- 
 

arivo,

 in 
 the 
 district 
 of 
 Yalalafotsy, 
 60 
 miles 
 west 
 of 
 the 
 capital. 
 

It

 is 
 a 
 compact 
 rock 
 composed 
 of 
 augite, 
 felspar, 
 olivine, 
 and 
 

magnetite.

The

 most 
 abundant 
 constituent 
 is 
 augite. 
 This 
 mineral 
 occurs 
 

in

 small, 
 sharply 
 contoured, 
 prismatic 
 crystals 
 of 
 a 
 pale 
 brown 
 colour. 
 

Under

 the 
 microscope 
 the 
 crystals 
 give 
 lath-shaped 
 sections 
 with 
 

pyramidal

 terminations. 
 Twinning 
 parallel 
 to 
 (100) 
 is 
 frequent. 
 

In

 several 
 cases 
 it 
 is 
 polysynthetic, 
 as 
 in 
 the 
 triclinic 
 felspars. 
 The 
 

extinction-angle

 has 
 a 
 maximum 
 value 
 of 
 about 
 42°. 
 

These

 little 
 crystals 
 of 
 augite 
 are 
 imbedded 
 in 
 a 
 matrix 
 of 
 felspar. 
 

The

 latter 
 presents 
 no 
 trace 
 of 
 twin-striation. 
 In 
 places 
 it 
 is 
 slightly 
 

kaolinized.

 The 
 relation 
 between 
 the 
 felspar 
 and 
 augite 
 may 
 be 
 said 
 

to

 be 
 " 
 ophitic 
 ;" 
 but 
 it 
 differs 
 from 
 that 
 structure, 
 as 
 represented 
 in 
 

the

 dolerites, 
 in 
 that 
 in 
 the 
 case 
 under 
 consideration 
 the 
 felspar 
 

is

 moulded 
 upon 
 idiomorphic 
 crystals 
 of 
 augite, 
 instead 
 of 
 augite 
 

upon

 felspar, 
 as 
 in 
 the 
 dolerites. 
 Similar 
 relations 
 between 
 felspar 
 

and

 augite 
 have 
 been 
 described 
 by 
 Rosenbusch 
 * 
 in 
 reference 
 to 
 

vogesites,

 and 
 by 
 Teall 
 f 
 in 
 certain 
 Scotch 
 traps. 
 

The

 order 
 of 
 consolidation 
 in 
 the 
 Madagascar 
 rock 
 appears 
 to 
 

have

 been 
 as 
 follows 
 : 
 — 
 

f

 Magnetite. 
 

1st

 phase 
 -j 
 Olivine. 
 2nd 
 phase 
 . 
 . 
 Felspar. 
 

.

 Augite. 
 

b.

 Olivine- 
 Basalt 
 with 
 Hornblende 
 and 
 Biotite 
 in 
 PorpTiyritic 
 

Crystals,

 (ffoniblende-olivine-basalt.) 
 

A

 rock 
 of 
 this 
 composition 
 occurs 
 in 
 the 
 volcanic 
 region 
 west 
 of 
 

Lake

 Itasy. 
 Its 
 predominant 
 constituent 
 is 
 augite, 
 occurring 
 in 
 

large

 black 
 crystals, 
 imbedded 
 in 
 a 
 slightly 
 vesicular 
 ground-mass, 
 

the

 latter 
 being 
 made 
 up 
 of 
 microlites 
 of 
 felspar, 
 granules 
 of 
 olivine, 
 

augite

 and 
 iron-ore, 
 together 
 with 
 a 
 small 
 quantity 
 of 
 interstitial 
 

glass.

Under

 the 
 microscope 
 the 
 augite 
 appears 
 sometimes 
 in 
 irregular 
 

masses,

 sometimes 
 in 
 characteristic, 
 octagonal 
 sections, 
 bounded 
 by 
 the 
 

faces

 (110,100,010). 
 Included 
 material 
 is 
 abundant, 
 namely: 
 — 
 glass, 
 

magnetite,

 olivine, 
 and, 
 more 
 rarely, 
 a 
 microlite 
 of 
 brown 
 hornblende. 
 

 Die 
 massigen 
 Gesteine, 
 1887, 
 p. 
 316. 
 

t

 British 
 Petrography, 
 1888, 
 pp. 
 188, 
 194,