File:The Quarterly journal of the Geological Society of London (13937226491).jpg

350

I)K.

 E. 
 H. 
 HATCH 
 ON 
 THE 
 CHAEACTEES 
 OE 
 EOCKS 
 

The

 crystals 
 almost 
 invariably 
 show 
 zonal 
 structure, 
 being 
 composed 
 

of

 successive 
 isomorphous 
 layers, 
 in 
 which 
 there 
 is 
 a 
 gradually 
 in- 
 

creasing

 difference 
 in 
 chemical 
 composition, 
 directly 
 influencing 
 the 
 

position

 of 
 the 
 axes 
 of 
 elasticity: 
 the 
 extinction 
 between 
 crossed 
 

nicols

 is 
 consequently 
 not 
 uniform 
 ; 
 it 
 takes 
 the 
 form 
 of 
 a 
 dark 
 

shadow,

 which 
 on 
 rotation 
 of 
 the 
 section 
 moves 
 from 
 the 
 centre 
 

outwards

 towards 
 the 
 periphery. 
 In 
 one 
 instance 
 a 
 difference 
 of 
 as 
 

much

 as 
 20° 
 was 
 measured 
 between 
 the 
 peripheral 
 layer 
 and 
 the 
 

central

 portion. 
 Even 
 in 
 ordinary 
 light 
 a 
 difference 
 between 
 the 
 

peripheral

 and 
 central 
 portions 
 can 
 sometimes 
 be 
 distinguished, 
 

the

 former 
 being 
 of 
 a 
 deeper 
 tint 
 than 
 the 
 latter. 
 In 
 sections 
 

parallel

 to 
 the 
 vertical 
 axis 
 the 
 central 
 portion 
 often 
 simulates 
 the 
 

shape

 of 
 an 
 hour-glass 
 (see 
 fig. 
 6). 
 This 
 structure 
 has 
 been 
 

explained

 in 
 the 
 following 
 way 
 : 
 — 
 During 
 the 
 consolidation 
 of 
 the 
 

rock

 the 
 augite 
 first 
 separates 
 in 
 skeleton-crystals 
 of 
 the 
 shape 
 of 
 

an

 hour-glass. 
 Later 
 on 
 the 
 depressions 
 in 
 the 
 sides 
 of 
 these 
 become 
 

filled

 in 
 by 
 augitic 
 material 
 of 
 a 
 slightly 
 different 
 composition 
 and 
 

possessing,

 in 
 consequence, 
 an 
 optical 
 character 
 deviating 
 from 
 that 
 of 
 

the

 first-formed 
 central 
 portion. 
 A 
 curious 
 instance 
 of 
 zoning 
 in 
 

augite

 is 
 shown 
 in 
 fig. 
 8. 
 Twinning 
 parallel 
 to 
 the 
 orthopinacoid 
 

Fig.

 6. 
 — 
 Hour-glass 
 Fig. 
 7.— 
 Augite 
 with 
 

structure

 in 
 Augite. 
 glass-inclusion. 
 

Fig.

 8. 
 — 
 Zoned 
 Augite. 
 

i

r

is

 of 
 frequent 
 occurrence. 
 In 
 some 
 cases 
 this 
 is 
 polysynthetic, 
 

as

 in 
 triclinic 
 felspar 
 (see 
 fig. 
 9) 
 ; 
 in 
 one 
 case 
 two 
 twins 
 are 
 united 
 

along

 a 
 common 
 face 
 (fig. 
 10). 
 

Among

 the 
 earliest 
 minerals 
 to 
 separate 
 were 
 the 
 hornblende 
 and 
 

mica.

 Of 
 these 
 minerals, 
 however, 
 the 
 major 
 portion 
 has 
 undergone 
 

resorption

 into 
 the 
 molten 
 magma 
 during 
 the 
 later 
 stages 
 of 
 consoli- 
 

dation.

 In 
 some 
 cases 
 there 
 remain 
 small 
 fragments, 
 encircled 
 

by

 a 
 broad 
 zone 
 of 
 the 
 products 
 of 
 fusion 
 .Schmelzzone). 
 In 
 

the

 case 
 of 
 the 
 hornblende 
 this 
 secondary 
 material 
 usually 
 retains 
 

perfectly

 the 
 original 
 form 
 of 
 the 
 crystals 
 after 
 which 
 it 
 is 
 pseudo- 
 

morphous.

 It 
 consists 
 of: 
 1, 
 opaque 
 particles 
 (magnetite); 
 2,