456 S. ALLPOKT ON PITCHSIONES AND PERL1TES FKOM 



as distinct and unquestionable as in any of the more recent glassy- 

 rocks. Fig. 6 represents a portion of a thin slice of one of these 

 ancient perlites ; and beside it I have placed an equally careful 

 drawing of a typical perlite from Schemnitz (sec fig. 5). The older 

 rock has undergone a considerable amount of alteration ; and that 

 constitutes the only perceptible difference between them. In some 

 of the Meissen pitchstones the perlitic structure is nearly or quite 

 absent, while in others it is well developed ; and it fortunately 

 happens that specimens occur in various stages of alteration. 



In one of the red varieties the colour of the mass is due to the in- 

 filtration of bright red ferric oxide, which has followed the lines of 

 fissure, and has also stained the glass for a short distance on each 

 side, as represented by the light shade in fig. 1. In the same spe- 

 cimen, however, a yellowish-brown substance here and there takes 

 the place of the red oxide, and shows a marked tendency to aggre- 

 gation in little spherical nodules. In a brown variety from the 

 same locality, a pale brown flocculent substance is alone present, and 

 has invaded the glass in the same manner but to a far greater 

 extent than in the preceding examples ; the parts permeated by it 

 have a distinct action on polarized light ; and it is quite evident that 

 a further extension of the process of alteration w T ould impart to the 

 mass a pseudo-felsitic aspect. 



The kind of alteration here described has clearly been produced 

 by chemical action ; and it has followed precisely the same course as 

 that which has so frequently converted fractured crystals of olivine 

 into serpentinous pseudomorphs. 



In the ancient perlite a similar process has been in operation, and 

 has produced the appearance represented in fig. 6. The shaded parts 

 indicate the presence of a yellowish-green substance, which accom- 

 panies the lines of fissure and has invaded the glass on each side. 



Devitrification of the glassy magma. — In addition to the chemical 

 alteration just described, the original glassy base of these old rocks 

 has also undergone certain molecular changes which it is highly 

 important to notice. A slight examination of the two thin slices 

 represented in figs. 5 & 6 suffices to show the identity of their 

 general character as seen in ordinary light ; but when placed under 

 the polarizing microscope between crossed prisms, it is at once seen 

 that the matrix of the Schemnitz rock remains dark, while that of 

 the older one transmits light in many places, and the field of view 

 exhibits an irregular mosaic of light and dark grains. 



On rotating the object, some of the dark grains may then be seen 

 to transmit light in certain positions ; but the greater number always 

 remain dark; and it becomes evident that the mass consists of a 

 homogeneous glass with numerous doubly refracting patches 

 disseminated through it. The extent to which the two substances 

 prevail varies considerably in different parts of the rock : in some of 

 the highly altered spherulitic varieties there may be seen a perfect 

 mosaic of varying pale shades of colour, while in others the glassy 

 substance predominates. The peculiar character of the doubly 

 refracting portion of the base is extremely well shown when the 

 axes of the ISficols are inclined to each other ; by a slight rotation of 



