256 H. P. CUSHING—SYENITE-PORPHERY DIKKS. 
EXPLANATION OF PLATE 
Puate 17.—Thin-sections of Material from Syenite-porphyry Dikes 
Figure 1.—Intergrowth of orthoclase and albite shown in the center. Dike 25, 
X nicols, X 67. 
Figure 2.—Portion of a large Carlsbad twin, occupying the entire field, showing 
intergrowth of albite and orthoclase, with two crossing sets of albite - 
spindles, X nicols, X 85. 
Figure 3.—Portion of large phenocryst from dike 3, showing intergrowth of same, 
x nicols, X 67. 
Ficure 4.—Flow structure, groundmass of dike 28, X nicols, X 67. 
Figure 5. —Flow structure around phenocryst. Dike 27, x nicols, x 22. 
Figure 6.—Thin-section of material from dike 31, with parallel nicols, to show the 
considerable amount of biotite and hornblende in the groundmass, 
feldspar white, biotite and hornblende (mainly the former) black. 
The white area to the right is a feldspar phenocryst, x nicols X 67. 
