SENFT ON ROCKS. V 



rocks from which they derive their origin. Chiefly found in the 

 immediate vicinity of extinct and active volcanos. 



1 . Light-grey, yellow, rust-brown, crystalline, or slaggy trachyte- 

 mass, in which nodules or fragments of trachyte, or pebbles, of dif- 

 ferent sizes are closely impacted : compact and hard. [Group 1 . 

 Trachytic. Trachyte-breccia .] 



2. Reddish-brown, grey- white, grey-brown, often bluish, yellowish, 

 or green-spotted or striped felsite-mass, in which either fragments, 

 or pea- or millet-sized grains of felsite-porphyry are imbedded. 

 [Group 2. Porphyrinic. Porphyry-breccia and porphyry-sandstone.] 



3. Greyish-green, dirty or blackish-green diabase-mass, in which 

 various-sized, often very large angular or rounded fragments of dia- 

 base, aphanite, or diabase-porphyry lie imbedded. [Group 3. Dia- 

 basitic. Diabase-breccia and diabase-sandstone. ~\ 



4. Impure black, porous, spongy, or slaggy tenacious melaphyre- 

 mass, in which large and small fragments of varieties of melaphyre 

 lie imbedded. [Group 4. Melaphyre-breccia.~\ 



5. Blackish-grey, also grey-brownish, slaggy or crystalline, com- 

 pact, more or less hard basalt-mass, in which lumps of dolerite and 

 basalt, and occasionally of limestone and sandstone, are included. 

 [Group 5. Basaltic breccia and doleritic breccia^] 



II. Order. The cement consists of quartz, limestone, dolomites, or 

 iron-ore ; crystalline. These fragmentary rocks are generally 

 stratified. 



1 . Quartz- or hornstone-matrix, in which angular, variously-sized 

 fragments or grains of common quartz, kieselschiefer (lydite), or 

 flint lie imbedded. [Group 6. Siliceous and lydian breccias, flint- 

 conglomerate, and arkose. 1 ^ 



2. Calcareous or dolomitic, crystalline, granular, or dense matrix, 

 in which more or less angular fragments or pebbles of limestone, 

 dolomite, or stinkstone are imbedded. [Group 1 . Calcareous breccia, 

 oolitic limestone in part, and dolomite- and stinkstone-breccias.~\ 



3. Matrix dense, consisting of brown or red iron-ore, in which 

 angular fragments or grains of different varieties of iron-ore, princi- 

 pally magnetic iron, iron-glance, iron-mica-schist, are imbedded. 

 [Group 8. Iron rocks. Tapanhoacanga.~] 



II. Hemiclastic Eocks. 



I. Order. Tufas, which, besides indistinct rock-fragments, enclose 

 numerous crystalline minerals, chiefly mica, augite, hornblende, 

 leucite, and magnetic iron. 



1. Grey, whitish, yellowish, brownish, generally soft and earthy 

 matrix. Contents : small and often indistinct fragments of trachyte, 

 phonolite, or pumice, and well-preserved crystals of sanidine, horn- 



