TERTIARY BASIC DIKE ROCKS. 
91 
volcanic rocks, but occasionally cut the granites. As in the case of the phonolites, 
there is seen a tendency toward radiation from the center of the eruptive area. 
When fresh they are nearly or quite black rocks, with conchoidal fracture, and vary 
in granularity a from fine-grained porphyritic to aphanitic. Their mineralogical 
composition is such that they readily undergo alteration and with the exception of 
the northeasterly dike which outcrops in the main street of Altman they are not 
conspicuous at the surface. This decomposition is not limited to the vicinity of the 
surface, but frequently extends to depths of over 1,000 feet, changing the rock in 
extreme cases to a soft and clayey greenish-gray crumbling mass, while less decom¬ 
posed portions vary from light to dark gray or greenish and are more or less porous 
through the removal of some of the constituents. 
These rocks are locally known as basalts and have usually been mentioned as 
such in the literature. On the basis of partially decomposed material, Cross 
made two divisions which he called plagioclase basalt and nepheline basalt, respec¬ 
tively. To these Stevens added limburgite* and tephrite. c Careful examination 
of specimens collected from all parts of the district establishes three definite rock 
types which while outwardly resembling basalts are because of their notable 
content of alkalies really not basalts at all. They are trachjMolerite, vogesite, 
and monchiquite. For various reasons it has been found impracticable to separate 
them on the map, where they appear as “basic dikes.” In order to distinguish the 
first-named type from the rock of Bull Cliff, it will be designated as the Isabella 
trachydolerite. 
TRACHYDOLERITE (ISABELLA TYPE). 
This group is exemplified by the dike in the Isabella mine, which can be traced 
through the town of Altman nearly to the summit of Bull Hill and extends north¬ 
ward through the Block 8 mine to Grassy Creek. It is also represented by the 
Dolly Varden dike, by a dike crossing the Gold Bond property, one in the Ajax 
mine, and several in the Portland mine, and by an inclined dike in the Mary 
McKinney mine. Several dikes, such as those in the Blue Bird and Midget mines, 
though much decomposed, probably also belong to this type. 
These rocks are distinctly phanerocrystalline, but of fine grain, and consist of 
plagioclase, pyroxene, and olivine, with smaller amounts of orthoclase and analcite, 
with or without a glass basis. As accessory or occasional constituents, occur apatite, 
magnetite, biotite, and hornblende. The plagioclase ranges from andesine to 
bytownite, inclusive, and though more than one species is sometimes contained in 
the same specimen, labradorite is most common. It occurs in tabular plates, but 
more frequently as laths, from over a millimeter in length down to minute microlites. 
It is seldom more than hypidiomorpliic, but in a few cases attains almost perfect 
form. It occurs at times only as phenocrysts, at others both as phenocrysts and 
in the groundmass, while not infrequently it is confined wholly to the groundmass. 
Arrangement in zones of not very different composition may often be observed, and 
occasionally the phenocrysts are surrounded by a narrow, irregular, clear zone of 
a Quantitative Classification of Igneous Rocks, Chicago, 1903, p. 154. 
b Trans. Am. Inst. Min. Eng., vol. 30, 1901, pp. 759-764. 
c Idem, voi. 33, 1903, p. 687. 
