CONTENTS. 
69 
Page. 
(b.) Consideration of magmas during their eruption _ 100 
(1.) Changes of temperature and pressure_ 101 
(2.) Temperature of the conduit_;-- 103 
(3.) Size of the conduit_ 103 
(4.) Changes in the velocity of eruption_ 104 
— acceleration and retardation of crystallization_ 104 
— resorption of crystals__ 104 
— order of resorption_ 105 
(c.) Rate of cooling and rock structures _ 105 
Relative influence of the various causes of crystallization _ 106 
(1.) Cooling and time (rate of cooling)___ 106 
(2.) Chemical composition___ 107 
(3.) Mineralizers_ 107 
(4.) Pressure_ 108 
— apparent lack of influence_ 108 
— identity of minerals formed under different pressures-. 108 
— identity of rocks at different altitudes__ 108 
— absence of dimorphism among the rock-making min¬ 
erals, except tridymite (which may be due to miner¬ 
alizers) _ 108 
— optical anomalies probably due to changes of tempera¬ 
ture _ 108 
Condition of rock magmas previous to crystallization _ 109 
— gas bubbles in glass inclusions__ 109 
— molten, fluid condition of the magmas___ 109 
Resume _ 109 
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