458 S. ALLPOKT ON PITCHSTONES AND PERLITES FROIT 



L, II. Ancient Pitchstone ; analyzed by J. A. Phillips. 



III. Perlite, from Hlinyik, near Schemnitz ; analyzed by Yon 

 Sommaruga, quoted by J. Koth *. 



IY. Pearlstone (Hungary) ; analyzed by itammelsberg f. 



An inspection of a list of analyses shows that there is quite as 

 close an agreement between these examples of ancient and Tertiary 

 perlites as can be found in a series of the latter only ; and as this 

 similarity of chemical composition exists notwithstanding the altera- 

 tion to which the older rocks have been subjected, it may be inferred 

 that in this instance, as in others, the changes have been almost en- 

 tirely molecular, little or nothing having been taken from or added 

 to the mass. 



Intimately associated with the characteristic devitrified pitchstone 

 there occurs a hard compact variety of a dark red colour and semi- 

 vitreous aspect, with a subconchoidal fracture and sharp cutting- 

 edges. Some specimens show a banded structure, even to the 

 naked eye ; while others, apparently amorphous, exhibit under the 

 microscope most interesting examples of fluidal structure, but with 

 no indications of perlitic or spherulitic formations. These also 

 appear to consist of devitrified glass, and are found to pass into 

 masses of ordinary " hornstone " or " compact felspar," which were 

 probably never in a vitreous condition. 



A close examination of very thin slices shows that the red colour 

 of these rocks is entirely due to the diffusion of hydrous ferric oxide 

 through the mass j a magnifying power of 800 enables one to per- 

 ceive that the matrix consists of a colourless glassy substance 

 crowded in parts with minute yellowish-red specks, which have the 

 appearance of a fine dust even under the highest powers. In many 

 cases the colouring-matter clearly has its origin in minute ochreous 

 patches presenting crystalline forms with ragged outlines, from 

 which the red stain has spread in all directions. 



Indurated Volcanic Agglomerates, and Ashes. 



An examination of the stratified fragmental rocks of Lawrence 

 Hill and the Wrekin leaves no room for doubt as to their real 

 character. 



The various beds consist for the most part of a breccia composed 

 of small angular and slightly rounded fragments of compact red 

 felsite and altered pitchstone, quite similar to those forming the 

 masses with which they are associated ; these, together with other 

 materials, have been cemented together, and now form an extremely 

 hard rock, which frequently exhibits manifest signs of subsequent 

 alteration. Fragments of larger size, however, are not uncommon ; 

 and in one of the coarser ash-beds a block of beautiful sphe- 

 rulitic pitchstone, 8 inches in diameter, was found imbedded in 

 the mass. A vertical slice of one of the finer ash-beds exhibits 

 under the microscope a series of thin layers composed of angular 



* Beitrage zur Petrographie. Berlin, 1869. - 



t Die Gesteins-Analysen. J. Eoth. Berlin, 1861. 



