244 H. P. CUSHING—SYENITE-PORPHYRY DIKES. 
become greenish, and in one dike are of a pronounced red-violet hue, a 
color which the thin-section shows to be due to hematite infiltration 
along the cleavage cracks. The other colors are the result of slight alter- 
ation or due to inclusions. 
The narrower dikes are dense, hard rocks with pronounced conchoidal 
fracture and aphanitic appearance. The larger dikes are equally hard 
and firm but coarser, the holocrystalline character of the groundmass 
being at once apparent to the eye. Some are of red color, others much 
darker, gray to black, with often a greenish tinge when slightly altered. 
The narrow dikes are more commonly red and the wider ones dark, but 
exceptions occur in each case. In the largest dikes the grain is suffi-- 
ciently coarse to permit both the feldspar and the biotite to show their 
own colors. 
In the case of dike 9, porphyritic biotite is common as well as feldspar, 
This is, however, true of this dike alone. 
MINERAL CONTENT AND STRUCTURE 
Order of abundance-—Under the microscope these rocks are found to 
contain the following minerals, arranged in order of abundance: Micro- 
perthite, biotite, magnetite, specular hematite, hornblende, quartz, albite, 
orthoclase, microcline, apatite, and titanite, with secondary chlorite, cal- 
cite, muscovite, epidote, and hematite. Microperthite and chlorite are 
the only two minerals present in all the dikes. 
Microperthite.—The feldspar phenocrysts found in most of the dikes 
occur either singly or in clumps or bunches variously intergrown and 
oriented with respect to one another. They invariably show twinning 
after the Carlsbad law. In habit they are quite diverse, but ordinarily 
are either rather stout prisms, with their greatest elongation parallel 
to the vertical axis, or are thin-tabular, parallel to the clinopinacoid. 
They are bounded by the planes P, MW, T,/, and «. Their boundaries 
are rough and somewhat irregular, with the adjoining groundmass firmly 
attached, giving the impression of a slight corrosive action by the latter. 
Carlsbad twinning is also universal in the groundmass feldspars. These 
are sometimes lath-shaped, sometimes of stout habit. They always con- 
stitute the greater part of the groundmass. 
Nearly all the feldspar shows the minute intergrowth of two feldspar 
species to which the term microperthite is applied. The intergrowths 
are of a quite varied character, ranging from a regular spindle arrange- 
ment, to which the name is sometimes restricted, to an irregular patchy 
mottling. The more common arrangement is illustrated in plate 17, 
figure 1. An unusual method is shown in figure 2, there being two sets 
of albite lamelle which cross one another, producing a mesh structure. 
