20 GEOLO&ICAL ilEMOIRS. 



On YiTKEOUS and Semivitreotjs Eocks. By Dr. F. Ziekel. 



[Proe, Imp. Greol. Institute, Vienna, March 3, 1868.] 



The rocks submitted to microscopic iDvestigation by the author 

 came from the Trachytic regions of Hungary, the Euganeans, Ice- 

 landj St. Paul, and ]N"ew Zealand, with some porphyiies from Saxony. 

 The specimens exhibiting devitrification show distinct traces of the 

 passage of the rocks from the liquid to the solid state, thus affording 

 decisive proof that since their solidification no change has taken 

 place in the relative situations of even the minutest crystalline con- 

 stituents. They also contain minute tabular crystals of magnetic 

 iron-ore, prisms of augite or hornblende, and felspar in microscopic 

 crystals. Although sanidine is the most abundant of the felspathic 

 constituents of vitreous rocks, triclinic felspar is of frequent occur- 

 rence either in isolated crystals or associated with sanidine, and is 

 of much more general occurrence than is usually supposed. Both 

 felspars frequently include either vitreous or partially devitrified sub- 

 stances in accordance with those which surroand them, thus proving 

 that the crystals of felspar have crystallized out of the amorphous 

 paste which the rock originally consisted of. The vitreous substance 

 frequently penetrates into the interior of the crystals in the form of 

 ramifications. This is the case, also, with the quartz which some- 

 times occurs in certain varieties of pitchstone. 



Of the vitreous rocks obsidian is the most typical, although the 

 process of devitrification has commenced in it, as is exhibited in 

 its most advanced stage in pumice. In the latter the formation 

 of pores has reached its maximum ; but such cavities exist also in 

 obsidian, though they have not yet been found to contain liquid sub- 

 stances. The vitreous granules of perlite have been found to include 

 belonite and trichite ; but the crystalline results of devitrification are 

 quite difi'erent from the concentric structure exhibited by the granules 

 of perlite. The same may be said of the granules of spherulite and 

 the felspar and magnesian mica crystallized out of them. 



The trachytic porphp-ies generally present traces of progressive 

 devitrification. In the Iceland rocks sanidine is most abundant ; but 

 the existence of triclinic felspar has been carefully ascertained, and 

 its crystals have been found to contain numerous pores, generally 

 filled with gaseous substances, but sometimes with crystals of quartz. 



A variety of pitchstone from the Island of Arran presents a good 

 instance of vitrified and devitrified substances in crystals of quartz 

 having uniformly an hexagonal form. 



The older pitchstone (euritic pitchstone) is composed of a vitreous 

 substance with simple refraction, and of a double -refracting euritic 

 material, with scarcely any admixture of belonite,but interspersed with 

 sanidine (including vitreous material), trichnic felspar, quartz (with 

 numerous pores containing liquids), and black mica. The formation 

 of eurite must consequently have taken place at the moment of solidifi- 

 cation, and cannot be ascribed to a subsequent metamorphosis. The 

 connexion between pitchstone and euritic porphyry, which has long 

 been admitted on geological groimds, is thus confii^med by the micro- 

 scopic investigation of the specimens described by Dr. Zirkel, which 

 were obtained from the district near j!J.eissen, Saxony. [Cofis^t M.] 



