178 WILLIAM J. MILLER 
the diabasic texture. Obviously, examples of this kind would not 
be common. 
The acidic (syenitic) border phase of stock No. 1 (see map) and 
represented by No. 10 of Table I might be explained on the basis of 
simple marginal assimilation, though there appears to be no good 
reason against explaining it as due to the assimilation of blocks 
of country rock stoped from the walls of the stock chamber. In 
accordance with Daly’s hypothesis the blocks of country rock 
would have been derived from a level some distance above the 
present observed contact but, because of the low specific gravity 
of the country rock (granite) as compared with the molten gabbro, 
and also because of the superheated condition of this gabbro magma 
(as proved by the vigorous contact action on the granite), those 
blocks of country rock would not have sunk very far in the magma 
before they became thoroughly assimilated. 
SIGNIFICANCE OF THE MORE ACIDIC STOCKS 
Five or six of the stocks are composed of rocks more acidic 
than gabbro. Fine examples are: Stock No. 3 (see map); 13 
miles northeast of Pottersville; # of a mile south-southeast of 
South Horicon; and 14 miles southwest of The Glen. In fact 
every gradation may be found from stocks of basic gabbro, through - 
acid gabbro and diorite, to syenite. In the earlier stage of very 
active intrusion the invading magma was more thoroughly molten, 
and as the blocks of country rock were stoped off they sank in the | 
magma and became completely dissolved and diffused. Since 
the country rock was almost always granite, syenite, or gneiss, the 
‘magma became more and more acidic. A magma thus formed 
has been styled a “‘syntectic’” magma. The amount of superheat 
required to dissolve the blocks would not be great because, as 
Daly has pointed out, there is plenty of proof that molten basic 
rock “‘even slightly superheated will dissolve fragments of gneiss 
and allied rocks. The mutual solution of two contrasted silicate 
mixtures takes place at a certain temperature which is lower than 
the melting-point of either one.’? According to the specific 
t Amer. Jour. Sci., XXVI (1908), 36. 
