Page:Popular Science Monthly Volume 48.djvu/388

356 them is usually marked by a sort of thread, either continuous or discontinuous, which sometimes suggests a cylindrical cavity, but is more usually in the condition of scattered granulations. The nature of these included bodies can often be determined with precision; and to select one of the clearest cases, they may not infrequently be perceived acting energetically upon polarized light as if they were true crystals, while they show geometrical forms on larger or smaller parts of their contour. On closer examination it is possible to refer them to grains of pyroxene. In some of the filaments the axial inclusions are of a different character; their perceptibly spherical form and other features identify them with the gaseous bubbles frequently observed in rocks. The

matter which fills the cylindrical cavity, often as long as the filaments themselves, is also gaseous; and so likewise are the bubbles that may sometimes be seen by thousands in the vitreous scales, such as are abundantly represented in an angular plate near the edge of Fig. 2, Another point to be noticed is the way the filaments terminate that we are sure have not been broken after consolidation. They are very rarely drawn out to a fine point without bending, but usually they suffer a more or less abrupt curve or are done up into a knot or a loop, of which Fig, 3 indicates some common forms.

If we lightly shake these locks above a sheet of white paper, a fine dust will fall upon it in which the microscope detects among the finest filaments myriads of brownish, translucent, and sometimes transparent, vitreous pellets. Most frequently they are perfectly homogeneous, but they also often contain inclusions similar to those in the filaments. The perfection of their spherical form is not strictly in proportion to their diameter, as might be