OCR Text |
Show TEACHYTES OE SUB-ACID EOCKS. 107 Gkoup II.â€"TEACHYTES OE SUB-ACID EOCKSâ€"Continued. Characteristics. 3. Argilloid trachyte...... A rock of very clayey or earthy aspect, suggestive of thick slate ; very highly charged with ferritic matter, rendering it opaque in the thinnest sections; holding crystals of feldspar (orthoclase) and grains of magnetite, and seldom any other macroscopic mineral. The fracture is highly characteristic, there being no cleavage; hut the rock crumbles rather than splits. It is, impossible to strike off thin flakes. The fracture is very angular and irregular, though the ordinary coarseness of trachytes is not exhibited. It is a very voluminous rock in the plateaus and well distinguished. 4. Hyaline trachyte........ Trachytes having a fluidal texture, indicative of flowing in a viscous state, with very small, and sometimes few, and always poorly-developed crystals of feldspar. Mostly reddish or purplish; often with a brick-like texture; sometimes foliated and resonant (clink-stone); moderately vesicular. Often slightly quartziferous and approaching the rhyolites. Sub-group B.â€"Hornblendic Trachytes. 5. Hornblendic trachyte... This comprises most of those dark-colored varieties of coarse, harsh texture, exceedingly rough, though many are less so. Hornblende and magnetite are abundant, the former in well-developed prisms. The feldspars are less conspicuous than in the preceding varieties, but are really present in greater quantity, as shown by the microscope. Plagioclase very abundant. Iron gray is the usual color. 6. Augitic trachyte......... It seems doubtful whether this rock should be considered as anything more than a variety of the hornblendic sub-group. It is characterized by the presence of augite in place of hornblende. The varieties are usually finer grained than the hornblendic, and resemble more the augitic andesites, to which, indeed, they are so closely related that it is sometimes difficult to distinguish them. Magnetite abundant and some biotite. 7. Phonolite................ A rock in which nephelin takes the place of triclinic feldspar. Usually contains also orthoclase and some hornblende; resonant, foliated, and in the rockmass is generally laminated in a very peculiar and striking manner 8. Trachytic obsidian...... A wholly glassy or vitreous rock, having the normal constitution of trachyte. |