Gregory.] TRACHYTE OF CHAPMAN TOWNSHIP. 1 63 
project to a considerable distance into the other. One section shows 
a partly developed microperthite intergrowth. All these larger 
unstriated feldspars arc of the clear sanidine variety and have much 
the appearance of quartz; their edges and crystal outlines are well 
preserved, and only rarely are they altered or muddied by inclusions. 
Judging from the analysis, these arc probably soda orthoclases. Next 
to the feldspars, patches of chlorite occupy the most prominent place 
among the larger minerals. The chlorite is surrounded by a clear 
zone, which between nicols is seen to consist of feldspar laths. The 
arrangement of the groundmass points to the former presence of 
a mineral which has been replaced by the chlorite. From the shape 
of the chlorite areas, and because of the absence of any carbonates 
among the alteration products, which might have been expected if 
the original mineral had been hornblende or augite, this mineral is 
believed to have been biotite. A few pieces of an altered brown ish- 
green mineral may be biotite, or possibly hornblende. A little second- 
ary quartz, with indications of rhombohedral cleavage and clouded by 
inclusions, fills a few cavities and cracks in the rock. 
The groundmass is composed of stringy, ragged feldspar microlites, 
often bent and curved. The feldspars are largely un twinned, although 
Carlsbads are found with the halves unevenly developed. Among the 
plagioclases sections were found with both Carlsbad and albite twin- 
ning, which made possible their determination as albite by Michel 
Levy's method. When viewed with a low power the groundmass 
shows a general flow structure in the longitudinal direction of the 
flattened vesicles; but under higher powers the fluxional character is 
not very conspicuous, even around the larger feldspars. Yellowish 
cryptocrystalline aggregates found sparingly in the section may be 
devitrified glass. At present the whole slide is sprinkled with minute 
greenish-yellow dots, which may be considered as representing grains 
of some ferromagnesian mineral. 
Analysis. — The alkalies present in the rock were determined by Dr. 
C. H. Warren, of the Sheffield Scientific School, and found to be: 
CaO, 1.06; Na 2 0, 8.75; K 2 0, .20. The trifling amount of potash was 
unexpected, and to guard against error a second determination was 
made, but with the same result. As the analysis shows, soda is the 
predominating alkili, and it must be the chief component of the feld- 
spars. The rock itself falls into the group of soda trachytes or kera- 
tophyres. 
Rocks of this group are rare in America, and for this reason they 
are interesting and worthy of considerable at tent ion. even though their 
field occurrence has not been worked out in detail And complete anal- 
ysis has not been made. 
