EXPLANATION OF PLATE I. 
Fig. I. The puecililic structure in quartz-porphyry. As the thin sec- 
tion is revolved between crossed nicols the field breaks up into different 
areas, which extinguish differently, so that in any one position one area 
will give its maximum amount of light, another its greatest darkness, 
and others intermediate degrees. 
From No. 211, north side of Shepard mountain, three-fourths way vip 
from the base. Magnified 44 diameters. 
Fig. 2. Pseudospherulites with orthoclase nuclei. The microgranitic 
portion is composed of quartz and feldspar. 
From No. 410, the wall rock of the big diabase dyke at the "Tin 
Mines." Magnified 44 diameters. 
Fig. 3. The " poecilitic " structure in quartz-porphyry which is much 
more coarsely crystalline than that shown in fig. i. Three quartz crys- 
tals are represented, each of which has a very large surrounding area 
that really is a portion of the central crystal, but spotted over with feld- 
spar material. One of the crystals has a number of small but well 
formed orthoclase crystals projecting into it. The areas which do not 
have central crystals are similar to the others, and consist of a quartz 
ground-mass with the feldspathic material scattered through it. 
From No. 347. Magnified 30 diameters. 
Fig. 4. Augite with a secondary green hornblende rim around it; 
biotite with a hornblende crystal enclosed, and also with magnetite scat- 
tered regularly through it; plagioclase, and beautiful micropegmatite. 
From No. 235, a coarse-grained quartz-diabase. Magnified 30 diame- 
ters. 
Fig. 5. Porphyritic primary quartz, a, and secondary quartz, c, with a 
chloritic core. The primai-y quartz, a, has a reactionary rim around it 
of almost uniform breadth, the result of a corrosive action of the molten 
magma upon the quartz. 
From No. 336, a fine-grained diabase-porphyryte. Magnified 30 diame- 
ters. 
Fig. 6. Diabase porphyryte with glass carrying trichites and crystal 
skeletons. The plagioclase is shaded with parallel bi-oken lines; the 
augite with small crosses. 
From No. 415, taken at the big dyke at the " Tin Mines." Magnified 
44 diameters. "'♦ 
