File:The Quarterly journal of the Geological Society of London (13960201533).jpg

igneous

 rocks 
 after 
 their 
 consolidation. 
 185 
 

8.

 Bearing 
 of 
 these 
 considerations 
 on 
 the 
 problem 
 of 
 the 
 Origin 
 

of

 Foliation 
 in 
 the 
 Metamorphic 
 Hocks. 
 

It

 is 
 scarcely 
 necessary 
 to 
 point 
 out 
 the 
 important 
 "bearing 
 of 
 this 
 

principle

 of 
 the 
 growth 
 of 
 crystals 
 — 
 and 
 especially 
 of 
 felspar-crystals 
 

—

 in 
 solid 
 rock-masses 
 upon 
 the 
 great 
 question 
 of 
 the 
 origin 
 of 
 folia- 
 

tion

 in 
 rocks, 
 a 
 question 
 in 
 the 
 discussion 
 of 
 which 
 the 
 Fellows 
 of 
 

this

 Society 
 have 
 taken 
 so 
 prominent 
 a 
 part. 
 

That,

 as 
 the 
 result 
 of 
 contact-metamorphism, 
 many 
 well-defined 
 

mineral

 species 
 are 
 developed 
 in 
 the 
 midst 
 of 
 solid 
 rocks, 
 the 
 crystals 
 

growing

 at 
 the 
 expense 
 of 
 and 
 deriving 
 their 
 materials 
 from 
 the 
 

surrounding

 detrital 
 fragments, 
 has 
 long 
 been 
 recognized. 
 These 
 

phenomena

 find 
 many 
 beautiful 
 illustrations 
 in 
 the 
 so-called 
 "spotted 
 

schists."

 I 
 hope 
 to 
 be 
 able 
 to 
 show, 
 on 
 a 
 future 
 occasion, 
 how 
 large 
 

a

 part 
 a 
 similar 
 action 
 plays 
 in 
 producing 
 the 
 characteristic 
 structures 
 

of

 many 
 fresh 
 and 
 apparently 
 unaltered 
 igneous 
 rocks. 
 

The

 founders 
 of 
 the 
 theory 
 of 
 dynamo-metamorphism 
 — 
 Scrope 
 

and

 Darwin 
 — 
 very 
 clearly 
 perceived 
 that 
 in 
 the 
 study 
 of 
 igneous 
 

rock-masses

 which 
 have 
 been 
 subjected 
 to 
 movements 
 and 
 internal 
 

stresses,

 we 
 find 
 alike 
 the 
 clearest 
 analogies 
 and 
 the 
 simplest 
 and 
 

most

 readily 
 studied 
 examples 
 of 
 the 
 processes 
 which 
 go 
 on 
 during 
 

the

 production 
 of 
 foliation 
 in 
 rocks. 
 Those 
 who 
 have 
 done 
 most 
 

towards

 establishing 
 the 
 theory 
 on 
 its 
 present 
 firm 
 basis, 
 by 
 tracing 
 

with

 the 
 aid 
 of 
 the 
 microscope 
 the 
 actual 
 changes 
 which 
 the 
 minerals 
 of 
 

rocks

 undergo 
 while 
 the 
 development 
 of 
 foliation 
 is 
 in 
 progress 
 — 
 and 
 

I

 especially 
 refer 
 to 
 the 
 beautiful 
 researches 
 of 
 Lossen 
 and 
 Lehmann 
 

and

 those 
 of 
 students 
 of 
 petrography 
 who, 
 like 
 Dr. 
 Reusch 
 in 
 

Scandinavia,

 Prof. 
 Williams 
 in 
 America, 
 and 
 Mr. 
 Teallin 
 this 
 country, 
 

have

 sought 
 to 
 follow 
 in 
 their 
 steps 
 — 
 have 
 skilfully 
 and 
 patiently 
 

pursued

 the 
 same 
 methods. 
 

Charles

 Darwin 
 was 
 able 
 to 
 show, 
 by 
 the 
 aid 
 of 
 a 
 pocket-lens 
 and 
 

blowpipe

 only, 
 that 
 in 
 a 
 noncrystalline 
 lava 
 in 
 the 
 Island 
 of 
 

Ascension

 which 
 had 
 been 
 subjected 
 to 
 powerful 
 internal 
 mechanical 
 

stresses,

 the 
 felspar, 
 augite, 
 and 
 magnetite 
 had 
 separated 
 from 
 the 
 

glassy

 mass 
 in 
 distinct 
 and 
 parallel 
 folia 
 *. 
 He 
 clearly 
 perceived, 
 

what

 some 
 in 
 more 
 recent 
 years 
 have 
 quite 
 failed 
 to 
 realize, 
 that 
 

mechanical

 force 
 'per 
 se 
 is 
 wholly 
 incompetent 
 to 
 produce 
 a 
 true 
 

foliation

 in 
 rocks 
 ; 
 but 
 that 
 the 
 mechanical 
 force 
 becomes 
 really 
 

effective

 by 
 determining 
 and 
 controlling 
 the 
 operations 
 of 
 chemical 
 

affinity

 and 
 crystallization; 
 and 
 it 
 is 
 by 
 these 
 that 
 the 
 metamor- 
 

phoses

 of 
 the 
 minerals 
 in 
 rocks 
 are 
 brought 
 about 
 which 
 result 
 in 
 

the

 development 
 of 
 the 
 foliated 
 structure. 
 

The

 more 
 carefully 
 these 
 metamorphoses 
 of 
 the 
 minerals 
 in 
 rocks 
 

are

 studied, 
 and 
 the 
 more 
 clearly 
 it 
 is 
 perceived 
 that 
 the 
 whole 
 

structure

 of 
 deep-seated 
 rocks 
 may 
 undergo 
 a 
 complete 
 transforma- 
 

tion,

 during 
 such 
 metamorphoses 
 of 
 their 
 constituent 
 minerals, 
 the 
 

less

 difficulty 
 will 
 be 
 felt 
 in 
 accepting 
 the 
 teachings 
 of 
 Scrope 
 and 
 

 Darwin's 
 ' 
 Volcanic 
 Islands,' 
 published 
 in 
 1844, 
 p. 
 56 
 (p. 
 64 
 of 
 reprint 
 issued 
 

in

 1876). 
 I 
 have 
 studied 
 Darwin's 
 original 
 specimens 
 with 
 the 
 microscope, 
 and 
 

can

 confirm 
 the 
 wonderful 
 accuracy 
 and 
 acumen 
 of 
 his