344 Herrick et al. on American Norytes and Gabbros. 
The magma consists of reddish or whitish amorphous material 
masking feldspar crystals of the first order. The diallage is 
equally altered but retains its green color and optical properties. 
The greater number of porphyritic crystals arc less obviously 
altered and preserve their outlines. Most of them are oligo- 
clase or orthoclase. The primary feldspar is often obviously 
of a higher basidity. Secondary quartz is also present. The 
most natural way to account for the peculiarities of this rock,, 
which occurs also on the ranges further west, would be to re- 
gard it as paragenous and formed below the surface by inter- 
action of the gabbi-o or diabase flows on an acid country rock,, 
in the process of which the crystallizing autogenous material 
became more or less mingled with secondary fused matter from 
the walls. This process would then be quite analogous to that 
involved in the formation of the porphyritic felsytes and para- 
genous diorytes of the north shore and elsewhere. The fact 
that the feldspars of the first order are more basic favors this 
view. Microscopic examination of the contact between the 
true gabbro and the red orthoclase-bearing variety seems to 
substantiate this theory also. The plagioclase on the gabbro 
side becomes less basic from diffusion from the contact, while 
those of the other side react as monoclinic and are iron-stained 
and greatly altered, yet even these often exhibit traces of pre- 
viously existing multiple twins. We are driven to the conclu- 
sion that the red rocks are the product of mingled autogenous 
gabbro with secondary acid material from the walls, and the 
interaction of the two under pressure and long -continued high 
temperature. The presence of potassium and sodium salts 
would explain the development of secondary quartz and the 
paragenesis of the feldspars. 
We have space to mention but one other member of this 
group. The garnetiferous gabb^'o from Granite Falls, A'linn. 
This is selected as still further illvistrating the paragenous ten- 
dency of gabbro. This is typical gabbro in its essentials, the 
greater part of the section consisting of a rather low-angled 
labradorite and abundant diallage of an intense green color. 
But the specimen is remarkable for large grouped garnets and 
free quartz, together with a very small amount of hornblende 
as alteration product of the diallage. The quartz and garnet 
