File:The Quarterly journal of the Geological Society of London (13937220822).jpg

352

 DE. 
 F. 
 H. 
 HATCH 
 ON 
 THE 
 CHARACTERS 
 OF 
 ROCKS 
 

sections

 present 
 the 
 cross- 
 jointing 
 parallel 
 to 
 the 
 basal 
 plane 
 and 
 the 
 

central

 accumulation 
 of 
 inclusions 
 in 
 the 
 direction 
 of 
 the 
 vertical 
 axis, 
 

characteristic

 for 
 apatite. 
 The 
 mineralogical 
 composition 
 of 
 the 
 

rock

 is 
 that 
 of 
 an 
 augite-andesite 
 ; 
 but 
 its 
 structure 
 is 
 basaltic, 
 

the

 felspar 
 being 
 confined 
 to 
 the 
 ground-mass, 
 while 
 the 
 augite 
 is 
 

porphyritic.

 With 
 the 
 exception 
 of 
 the 
 absence 
 of 
 olivine 
 it 
 differs 
 

in

 no 
 essential 
 respect 
 from 
 the 
 basalt 
 described 
 in 
 section 
 b. 
 

d.

 Basalt 
 with 
 Hornblende 
 as 
 a 
 Constituent 
 of 
 the 
 Ground-mass. 
 

The

 rock 
 placed 
 under 
 this 
 head 
 is 
 a 
 basalt 
 of 
 rather 
 unusual 
 

type.

 It 
 occurs 
 in 
 one 
 of 
 a 
 series 
 of 
 dykes 
 on 
 the 
 sea-coast 
 at 
 Am- 
 

bodimadiro,

 on 
 the 
 north-west 
 coast. 
 Originally 
 vesicular, 
 its 
 cavities 
 

have

 become 
 infilled 
 with 
 fibrous 
 zeolites 
 and 
 calcite. 
 The 
 powdered 
 

rock

 effervesces 
 strongly 
 with 
 hydrochloric 
 acid 
 ; 
 and 
 on 
 standing 
 in 
 

the

 cold, 
 gelatinous 
 silica 
 separates. 
 No 
 cubes 
 of 
 salt 
 being 
 formed 
 

in

 the 
 solution 
 when 
 evaporated, 
 the 
 zeolites 
 must 
 be 
 lime-zeolites 
 

(scolecite,

 phillipsite). 
 

The

 chief 
 porphyritic 
 constituent 
 of 
 the 
 rock 
 is 
 augite 
 in 
 nu- 
 

merous

 large 
 and 
 well-shaped 
 crystals, 
 which 
 are 
 partially 
 invaded 
 

by

 serpentinous 
 and 
 calcareous 
 alteration-products. 
 Plagioclase 
 also 
 

occurs

 porphyritically 
 ; 
 but 
 its 
 crystals 
 are 
 few 
 and 
 small. 
 

The

 ground-mass 
 is 
 a 
 plexus 
 of 
 felspar-laths, 
 between 
 which 
 are 
 

scattered

 numerous 
 small 
 crystals 
 and 
 needles 
 of 
 brown 
 hornblende 
 

and

 granules 
 of 
 magnetic 
 iron-ore. 
 Interstitial 
 glassy 
 matter 
 

could

 not 
 be 
 detected. 
 

In

 its 
 general 
 character 
 the 
 rock 
 much 
 resembles 
 a 
 tephrite 
 : 
 and 
 an 
 

attempt

 was 
 made 
 to 
 prove 
 the 
 presence 
 of 
 nepheline. 
 But 
 the 
 fact 
 

that

 the 
 hydrochloric-acid 
 solution 
 of 
 the 
 powdered 
 rock 
 gave 
 no 
 

cubes

 of 
 salt, 
 tends 
 to 
 negative 
 this 
 supposition. 
 After 
 etching 
 and 
 

treatment

 with 
 fuchsine, 
 however, 
 a 
 number 
 of 
 small 
 specks 
 retained 
 

the

 colour 
 ; 
 and 
 it 
 is 
 quite 
 possible 
 that 
 these 
 may 
 be 
 granules 
 of 
 

nepheline.

It

 is 
 an 
 interesting 
 fact 
 with 
 regard 
 to 
 this 
 rock 
 that, 
 while 
 augite 
 

appears

 as 
 a 
 porphyritic 
 constituent, 
 hornblende 
 is 
 present 
 only 
 

in

 the 
 ground-mass, 
 thus 
 reversing 
 the 
 usual 
 order 
 of 
 things. 
 

The

 occurrence 
 of 
 hornblende 
 as 
 a 
 constituent 
 of 
 the 
 ground-mass 
 

of

 basalts 
 has 
 been 
 mentioned 
 by 
 C. 
 Chelius 
 *. 
 

e.

 Felspar-free 
 Basalt 
 (Magma-basalt, 
 Boiicky). 
 

Belonging

 to 
 this 
 class 
 is 
 a 
 black, 
 compact, 
 semi-vitreous 
 rock, 
 

from

 a 
 lava-stream 
 near 
 the 
 village 
 of 
 Ambohipolo, 
 in 
 the 
 volcanic 
 

region

 to 
 the 
 west 
 of 
 Lake 
 Itasy. 
 

In

 thin 
 section 
 this 
 rock 
 appears 
 as 
 a 
 clear 
 brown 
 glass, 
 containing 
 

numerous

 microlites 
 and 
 crystals 
 of 
 augite 
 and 
 magnetite, 
 the 
 

latter

 in 
 square 
 and 
 lozenge-shaped 
 sections, 
 varying 
 in 
 diameter 
 

from

 -006 
 mm. 
 to 
 -086 
 mm. 
 Olivine 
 is 
 rare. 
 The 
 augite 
 is 
 pale 
 

 ' 
 Erlautervmgen 
 zu 
 Blatt 
 Messel 
 und 
 Blatt 
 Eossclorf 
 der 
 geolog. 
 Karte 
 

des

 Grossk. 
 Hessen.' 
 Darmstadt, 
 1886. 
 Eosenbusch, 
 Die 
 massigen 
 Gest. 
 

vol.

 ii. 
 p.