620 JOHN JOHNSTON AND PAUL NIGGLI 
ponents and the penetrating gases (fluorides, sulphides, etc.). 
Furthermore, the conditions of formation of the original rock are 
of importance, as indeed they are with any metamorphic process; 
for if these conditions were not very different from those which 
obtained at the period of metamorphic action, the amount of this 
action would be slight, because there would be either no tendency 
toward another equilibrium or only an instability of single phases. 
Usually in cases of contact metamorphism there is a small 
pneumatolytic addition of material, which may be temporary or 
permanent. If it consists essentially in a saturation with water 
vapor it will materially favor recrystallization. In the rock close 
to an eruptive contact there is often observed an increased con- 
centration of alkalies, especially of soda. In sediments—clays and 
shales—there is usually more potash than soda, a predominance 
which is correlated with the relative solubility or stability of 
potash- and soda-minerals, since clastic sediments are in large 
measure merely residues, the least soluble portions of the original 
material under the particular conditions to which it has been 
subject. Contact-metamorphosed shales and schists, on the other 
hand, often have a much larger soda-content than the unaltered 
material of the same horizon, as is illustrated by the following 
analyses of rocks in the Christiania region, as given by V. M. 
Goldschmidt. 
Unchanged Schists from the Same Locality 
Contact eee ee eee 
Metamorphic 
I 2 
SO, oid Gdiad ebesscuasenone dee 54-95 54-43 49.46 
pl O Panta acetate lahcaner ee ata hall Gusta Tima tlre eM ghar we © 0.89 
FW O ai acer senses dex auciena estate 16.32 15.93 19.44 
Nos O ee Rai alin ce cig iWaradia Wain l 2.95 \ oe Te 
COs 2 eee Rh es owe el eed agian pean ans 5.66 f \ 6.03 
IM BORN meen gaits ns dle gies ONTO) cei lige bakoeabsrone: O.II 
Mig © sch tai Rea een calm aa 4.89 3.50 4.68 
CPOE Rane aa a etaintig nak eae a 3.88 3.56 3.16 
IN ZO) is Ueto SIN ata ean en aceasta 5.56 0.74 1.55 
KG Oye oie Ge Alcea meat eget a 3.56 3-44 4.12 
Pe eeann Ponta Gata reyes secede ela anaealle- spe Teens Si 0.66 oeeeteue 
ossionupnition ae see ceeee ee 0.71 7.19 6.37 
d oP eae ee Seater Antal. ieeee renal TUE Damme acomab ree pete ula a oi 0.2 
COs eis SAGO Neal eae te || oN Teg Bes a a | UP 3.70 
99.79 97.87 IOI.17 
