24 
MUSEUM BULLETIN NO. 2. 
differentiated from a magma closely related in time and origin 
to the magma of the simple sills. 
Granting this, there may be two ways in which the granite 
may have originated :■ — 
(1) The granite (micropegmatite) may represent the simple 
fusion of the sediments into which the gabbro was intruded. 
The presence of a granite (micropegmatite) zone in the interior 
of some of the sills at once prevents this from being a working 
hypothesis. 
(2) The granite (micropegmatite) may represent the assimila- 
tion of blocks of the surrounding sediments which have been 
stoped off by the intruding gabbro. After the assimilation of 
the sediments, this syntectic differentiated under the action of 
gravity giving the mass a stratiform appearance. (Daly’s 
assimilation-differentiation theory) . 
This hypothesis is based on the idea of an intrusion of a 
homogeneous gabbroidal magma into sills of various thickness, 
and as Daly has pointed out 1 demands that “a great absolute 
amount of thermal energy be credited to a sill in which secondary 
granite has been formed; that the sill must be thick. Other 
things being equal, granite formed by mere differentiation 
should be found in sills of less thickness, though here again the 
thickness must be considerable. True granite with relations 
described in this paper has never been found in any intrusive 
sheet 500 feet or less in thickness. The assimilation-differen- 
tiation theory readily interprets the facts as due to the relatively 
enormous amount of heat required for the generation of the 
granite-granophyr zone, namely, the amount of heat character- 
istic of thick intrusive sheets.” 
Field work by the writer subsequent to the publication of the 
article containing the above extract, revealed the presence of 
sills 140 feet thick which contained 70 feet of granite (micro- 
pegmatite) at the upper contact. Applying the hypothesis 
propounded by Daly, it is necessary to suppose, in such a case, 
that a sheet of gabbro with a thickness of about 70 feet rifted 
off and absorbed an equal thickness of quartzose sediments; 
this credits the gabbro body with an amount of thermal energy 
R. A. Daly, Amer. Jour. Sci., vol. 20, 1906, p. 214. 
