Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/539

Rh When examined under the microscope in polarized light, with crossed prisms, the central red spot and its colourless border exhibit a perfectly distinct fibrous radial structure, the central disk still retains its bright red tint, and the colourless border appears of a pale grey, except where obscured by the arms of the black cross. Frequently, however, the red stain extends quite to the edge; and a fibrous red spherulite is then surrounded by a zone of homogeneous glass. Although the fibrous crystals usually radiate from a central point, there are not a few spherulites which exhibit two distinct modifications of this arrangement. In one the fibres are seen to radiate from several points surrounding a felsitic mass of irregular shape; the rays forming the different groups meet each other along diverging straight lines; and the whole is surrounded by a glassy ring. In other cases the spherulites are ellipsoidal, and the fibres usually radiate from a point near one extremity of the axis. Small crystals of felspar are frequently enclosed in the spherulites; but, precisely as in the Hungarian perlite previously mentioned (p. 453 and fig. 7), their position has no relation whatever to the radial crystallization of the substance by which they are surrounded; this is clearly seen in fig. 10, which shows two felspar crystals enclosed in a spherulite.

Another striking resemblance between the ancient and more recent examples is found in the fact that the transparent matrix in which the spherulites are enclosed frequently exhibits a perfectly distinct perlitic texture, as shown here and there in fig. 8, and is also crowded with streams of microliths, which pass straight through the spherulites, precisely as in the Kremnitz rock represented in fig. 7.

The microliths closely resemble the more recent belonites in size and shape; and even the singular and unmistakable trichites, with the same twisted and knotted forms, are abundant in some of my sections. One kind, consisting of strings of minute dots, are the most prevalent, and are precisely similar to those observed in some specimens of spherulitic pitchstone from Schemnitz.

Besides the felspar crystals just mentioned, others are scattered here and there through the matrix, and cause streams of belonites to bend round them. Orthoclase and plagioclase are both present, and are remarkably well preserved; the latter appears to predominate; and the crystals are often beautifully striated.

The rock just described appears to pass gradually into a variety from which the spherulites are absent, but which presents most excellent examples of perlitic structure.

The examples in my possession are of a rather dull yellowish-brown colour, and are slightly fissile in one direction. When examined with a lens, a freshly broken surface exhibits numerous small convex and concave faces; and when a thin slice is placed under the microscope a true perlitic structure is at once seen to be