22 GEOLOGICAL MEMOIRS. 



have been incorporated during the gradual increase of the crystals, 

 and which have become solidified into vitreous or lithoid cavities or 

 pores*. During refrigeration small acicular crystals of a black 

 substance have been deposited within the vitreous cavities, which, like 

 the water- cavities, generally include a vesicle. Transitions between 

 vitreous and lithoid cavities occur frequently, and in great variety, 

 together with empty cavities resulting from the development of 

 gaseous substances. The whole of these structures, only visible by 

 the aid of the microscope, may throw some light on the origin of the 

 rocks in question, being indicative of the simultaneous action of 

 water and vapours and of the igneous fusion of solid substances. In 

 nearly all varieties of granite the quartz includes myriads of crystals 

 of vitreous felspar, irregularly spread in every direction. A polarizing 

 apparatus connected with the microscope proves the small quartz- 

 granules of the granites to be a combination of minute crystals. The 

 fundamental mass of the euritic porphyries, showing three distinct 

 types, has proved to be really (as it has been generally supposed to 

 be) an intimate mixture of quartz and felspar ; the quartz in many 

 of them, however, is more prevalent than geologists generally are 

 ready to admit. The basalts, appearing to the naked eye merely as 

 a homogeneous mass, exhibit, under the microscope, their various 

 constituents, and,, by examining a great number of varieties, the 

 progress of decomposition, so interesting in these rocks, may be 

 followed through all its stages. The pitchstones, generally admitted 

 to be composed of an amorphous vitreous substance, have proved to 

 consist of a confused aggregate of innumerable acicular crystals, 

 imbedded in a comparatively small proportion of vitreous matrix, the 

 nature of which is still doubtful. Obsidians, cautiously heated with 

 hydrofluoric acid, exhibit microscopical acicular crystals, which may 

 be also seen, as shown by the late Prof. Leydolt, in artificial glass 

 when acted upon by the same acid. [Count M.] 



On the Cretaceous Strata of the Circles of Koniggratz and 

 Chrudim (S.E. Bohemia). By C. H. M. Paul. 



[Proceed. Imp. Geol. Instit. Vienna, January 20, 1863.] 



These strata have been shown by Professor Reuss to rest on the Gault, 

 and consequently they represent M. d'Orbigny's " Cenomanien " 

 and " Turonien." 



Their subdivisions are, in descending order: — 1. Planer ; 2. Quader 

 Marl ; 3. Quader Sandstone ; 4. Quader Conglomerate. 



The Planer (1) consists of thinly laminated, argillaceous (locally 

 calcareous) marls, with Inoceramus Cuvieri and /. planus (both 

 abundant) ; also Micraster cor-anguinum, Pecten membranaceous, 

 Nucula pectinata, Terebratulina chrysalis, and Tellina tenuissima. The 

 Quader Marl (2) may be further subdivided into («) an inferior, 



* Synonymous with the " stone-cavities " of Sorby; see Quart. Jo urn. Geol. 

 Soc. vol. xiv. p. 466. — Edit. 



