THE LOWER- SILURIAN DISTRICT OF SHROPSHIRE. 455 



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 predomi- 

 nate ; 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. 



Devitrified Perlite, 



The examples in my possession are of a rather dull yellowish- 

 brown colour, and are slightly fissile in one direction. When exa- 

 mined 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 



Q.J.G.S. No. 131. 2h 



