72 
IDDINGS. 
most to a conception of the interior of the earth, or even of 
the possible source of igneous rocks. It is intended rather 
to prepare the way for the discussion of such phenomena by 
considering those which lie at the foundation of an under¬ 
standing of the true nature of the rocks themselves. It will, 
therefore, attempt to present some of the most prominent and 
general facts connected with the crystallization of igneous 
rocks, and to show what conclusions as to the early condi¬ 
tion of these rocks may reasonably be drawn. 
The student of crystalline rocks should never lose sight of 
the fact that a great number of the rock-making minerals 
crystallize from molten magmas and from aqueous solutions 
without appreciable difference in their characters, and that 
metamorphic processes set in action by aqueous solutions, 
by heating, or by the pressure derived from great dynamical 
movements, may in some instances lead to results easily con¬ 
founded with the products of the direct crystallization of 
molten magmas. It is, therefore, necessary to be thoroughly 
acquainted with the geological occurrence of all rock-bodies 
concerning whose primary and unmetamorphosed condition 
there is any doubt. 
The present paper deals exclusively with the results of the 
original crystallization of molten magmas, so far as the writer 
is able to judge of them, and is, therefore, illustrated very 
largely from the writer’s observations. 
Part I.— The Phenomena of Crystallization. 
j RocJc Structures in General Terms .—It would be impossible 
in anything less than a treatise to do more than sketch as 
briefly as possible some of the general features of rock-crys¬ 
tallization as they have been observed by all students of 
microscopical petrography, dwelling, perhaps, at greater 
length upon some points that have specially presented them¬ 
selves to the writer in the course of his regular work. 
(1.) Lavas that reach the surface of the earth in a fluid 
condition consolidate upon cooling rapidly to a more or less 
