G. F. Becker—Teature of Massive Rocks. 55 
_hofen designated it as syenite. It continued to be thus classed 
until Professor Zirkel showed by microscopic examination that 
it is a plagioclase rock. These facts are cited to show that it is 
substantially a granular rock of the granitic type. So nearly 
as I can estimate, about 95 per cent of the mass is granular, 
the remainder consisting of more or less porphyritic matter. 
While the general character of the rock is everywhere the 
same, there are considerable and rapid changes in the composi- 
tion and the texture of the material from point to point; but the 
same varieties occur over and over again within short distances 
and are everywhere confined to substantially the same range. 
Thus during the summer of 1885 I studied with care the excel- 
lent and extensive exposures which had just been made on the 
3000-foot level of the Chollar mine. Here I collected every 
variety of the rock exposed, and of these six or eight were 
distinguishable either by coarseness of grain, or by a more or 
less pronounced porphyritic texture. I carried these specimens 
to the west wall of the croppings, 3,000 feet. above the same 
claim, or in the same vertical plane, and found neither any 
general difference nor any difficulty in matching each specimen 
from below by others from the surface. For the interval be- 
tween the croppings and the summit of the mountain similar 
statements hold true. There is throughout, no indication of a 
tendency to a change varying with depth below the summit. 
The bare, faulted slopes of Mt.. Davidson afford an admirable 
opportunity for a study of the relations which exist between 
the porphyritic and granular forms of diorite. Patches of por- 
phyritic rock are surrounded by granular material, and patches 
of granular matter are surrounded by a porphyritic rock. 
Neither one nor the other form inclusions. They resemble the 
dark spots so constantly met with in granite and, in innumer- 
able instances, show a transition from one structure to the 
other. In some cases this transition is rapid though unmis- 
takable, in others it is so gradual that it would be impossible 
to decide within some inches where to draw the line between 
the granular and porphyritic forms. Only a very small portion 
of this mass is micaceous. Here may be found a single flake 
of biotite, there a group of flakes fading out from a centre 
into the ordinary granitoid mass. 
It is manifest that where a spot which is a few inches in . 
diameter fades out into the surrounding granular mass, the 
material of which each is composed must have been subjected 
to identical physical conditions. Neithercan have chilled more 
rapidly than the other and the pressure on each must have been 
the same. But the differences between rocks can be due only 
to physical or chemical conditions. The porphyritic portions of 
this rock must therefore have a different composition from the 
