of Mixed Rocks. xix 



Genus IS. WITH AN UNDETERMINED BASE. 



Species 1. LAVA. 



A mixed or undetermined base, having evidently b^en 

 melted, often porous, with cavities for the greater part empty, 

 enveloping different minerals. 



Principal varieties. 



Basaltic L Black compact paste, with void cells which 



are more or less abundant. 



Tephrenic L The paste of an ash-grey colour, harsh to the 



touch, and porous. 

 (Tephrine, Delametherie. Lava of Vol vie.) 



ScoRiACEOUS L A black, grey, or reddish paste, with nume- 

 rous vesicles, &c 



PoRPHYRlTic L A vitreous or slightly lamellar paste envelop- 

 ing crystals of vitreous and fibious felspar. 



Pumice L A pumice paste enveloping vitreous felspar. 



Class III. THE AGGREGATED ROCKS. 



Character. Formed by mechanical aggregation ; the 

 cement or paste posterior to the parts it contains. 



Genus 1. THE CEMENTED ROCKS. 

 The parts united by a slightly apparent cement. 



Species 1. PSAMMITE. (Micaceous sandstone,— coal 

 grit,' — the greater part of grey wackes.) 



A granular rock, principally composed of small grains of 

 quartz mixed with other minerals, and united by a slightly 

 apparent cement of a different nature. 



Principal varieties. 

 QuARTZOSE Ps Middling sized grains of quartz are essen- 

 tially predominant, with tome disseminated 

 grains of felspar, &c. 

 (Remilly, near Dijon. Martes de Vayre, near 

 Clermont in Auvergne, — above Carlsbad 

 in Bohemia.) 



