TERTIARY VOLCANIC ROCKS-TRACHYDOLERITE. 
89 
Microscopical and chemical study shows that this rock belongs in Rosen- 
husch’s family of trachydolerites, which are defined as containing in a fine-grained 
groundmass phenocrysts of generally calcic plagioclase and of pyroxene, and often 
accompanied by sanidine, a mineral of the sodalite group, hornblende, and olivine. 
MINERALOGICAL CHARACTER. 
The essential minerals of this rock are orthoclase, plagioclase, pyroxene, 
biotite, and analcite, with abundant black iron ore and many crystals of apatite 
as accessories, and here and there a little glass. 
Phenocrysts of feldspar rarely exceed 0.5 mm. in greatest dimension and are 
always triclinic, belonging to the variaties oligoclase and andesine. Twinning is 
common and cleavage is particularly evident. Aside from prominent clinopina- 
coids, crystal faces are not well developed. 
Feldspar of the second generation is about equally divided between orthoclase 
and plagioclase the latter including albite, oligoclase, and andesine. They are 
usually developed as laths, as in the trachytes, but are often of irregular form. 
Pyroxene is very abundant, both as phenocrysts reaching 0.3 mm. in size, often 
anhedral, and as small lath-shaped microlites profusely scattered through the 
groundmass. The mineral is colorless or very pale yellowish-green and without 
noticeable pleochroism. The interference colors are moderate and the angle of 
extinction is large. The optical properties and the composition of the rock make 
it probable that this pyroxene is related to augite. 
The manner of occurrence of biotite is rather unusual. It forms small ragged 
grains of brown color and intense pleochroism and is closely associated with grains 
of iron ore. Elongated individuals of larger size, sometimes 2 to 3 mm. in length, 
show what is probably an unusual kind of resorption. Small grains of black iron 
ore, pyroxene, and a colorless mineral—probably feldspar—mingle throughout the 
mass with residual particles of the biotite still retaining the original orientation. 
At times the pyroxene grains arrange themselves according to crystallographic 
directions, and then the pyroxene and biotite individuals possess the vertical axis 
in common. The pyroxene resulting from the magmatic alteration of biotite 
appears to be identical in composition with the original pyroxene. 
Analcite occurs in variable amounts, as rounded patches 1 mm. or more in 
diameter, and more commonly as irregular interstitial grains between the other 
groundmass constituents. When in the larger patches, it holds abundant laths of 
pyroxene and feldspar arranged in the same parallel fluidal way as in the other 
parts of the rock. The mineral is uniformly colorless and clear, shows the low 
single refraction and characteristic rough surface, and is sensibly isotropic, though 
small areas of exceedingly faint polarization are occasionally seen. The possibility 
of this material being glass, as its occurrence suggests, is almost wholly excluded by 
its clearness and freshness in a rock which carries over 1 per cent of carbonate. No 
question can be raised, on the other hand, as to the primary character of the analcite. 
Apatite is common in crystals which range in size from very minute up to 
0.3 mm. They are nearly always crowded with the acicular inclusions described 
under the latite-phonolites, and consequently present a very striking pleochroism 
in brown and purple. 
