10 
MUSEUM BULLETIN NO. 27. 
rare, are of interest as representing the acid extreme reached in the 
differentiation of the magma, from which all the various rock types 
described above are believed to have separated in turn. 
ORIGIN OF THE MINERALS. 
The mode of origin of the serpentine (including chrysotile), chromite, 
brucite, hydromagnesite, etc., is well established or obvious and need 
not be discussed here. 
Many of the other minerals, however, occur in a somewhat unusual 
manner, which calls for an explanation. These are for the most part 
alumino-silicates rich in lime, such as diopside, vesuvianite, and grossularite. 
They are commonly found filling dyke-like fissures within the peridotite, 
or its altered equivalent, serpentine; these dykes sometimes consist 
entirely of compact granular aggregates of diopside, vesuvianite, or 
garnet crystals. 
The minerals just enumerated include some of the species most 
commonly formed in the contact metamorphism of impure limestones. 
In the present case, however, both the attitude of these deposits and the 
absence of limestone beds, preclude the possibility of their having 
originated in this way. The minerals do not occur as a contact phase, 
but as dykes or veins well within the basic igneous masses; and, more- 
over, the intruded sediments around the margins of the igneous masses 
do not include any limestone beds, but consist of Cambrian slates, 
sandstones, and quartzites. 
It becomes necessary, therefore, to seek for some other source for 
the lime and, at first sight, the igneous rocks with which the minerals are 
associated do not appear to be the source. The following figures taken 
from analyses by M. F, Connor, and quoted in Dresser’s report, give 
the percentage of lime in the principal rocks which make up the mass of 
the serpentine belt: 
Lime Content of Rocks. 
CaO percentage 
Rock 
Locality 
0-68 
Peridotite 
Near Black Lake station 
0*20 
Serpentine 
u a a 
15-20 
Pyroxenite 
Lot 40, range II, Garthby, Wolfe county 
6-80 
Diabase 
u u ii tt 
Thus the peridotite, which constitutes by far the major portion of 
the igneous masses as exposed in this area to-day, contains less than one 
per cent CaO. 
While the writers cannot claim to have made an exhaustive study 
in the field of the mode of occurrence of these minerals, they have had 
