214 Classification of Massive Rocks. 
with the intrusives, but since no direct connection has been traced 
between them and deeply buried rock masses, of which they may 
be regarded as a part, he decides to place them in a separate group 
until more knowledge of their relations has been obtained. 
The structure of the vein rocks resembles in some respects that 
of the intrusive rocks, and in others that of the effusives. Three 
well-marked types are recognizable: the granitic, the granite- 
porphyry, and the lamprophyre. Since these three types can be ` 
distinguished macroscopically, and are very characteristic, they are 
made use of to separate the vein rocks into three groups, which are 
in turn subdivided into families, according to mineralogical consti- 
tution, as in the class of intrusive rocks, 
II. A. THe Granitic Vern Rocks. 
The structure of the granitic vein rocks differs from that of the 
intrusive rocks, in that their individual constituents tend to become 
idiomorphic. In most cases this tendency has gone so far as to 
produce a rock, all of whose components are bounded by their own 
crystal outlines. They are then panidiomorphic-granular. An 
approach to the porphyritic development is sometimes the result of 
a repetition of conditions which allows of the separation of some of 
the constituents in two generations. 
The granitic vein rocks include :— 
1. APLITE, or muscovite granite, consisting of orthoclase, quartz 
and muscovite. 
(A) pegmatite is a coarse-grained aplite. 
(B) beresite is orthoclase-poor aplite. : 
II. B. THE GRANITE PORPHYRY GROUP. 
The structure characteristic of this group of rocks is the holo- 
crystalline-porphyritic. Their ground mass is a granular aggregate 
of crystalline minerals. The predominant porphyritic crystals are 
light in color, i.e., they are neither iron nor magnesia bearing. The 
occurrence of iron and magnesia-bearing minerals as porphyritic 
constituents is exceptional, 
The group is divided into ;— 
A. THE GRANITE PORPHYRIES. . 
The mineralogical composition of this family is the same as that 
