62 OUTl.l\!^"l KI ELD-GEOLOGY PARTI 



hate of lime, etc. The gn :ty of tin 



mainly composed of some felspar, or at least contain a 

 large percentage of that mineral, with such silicates as 

 hornblende, augite, olivine, biotite, and n 

 silica in the form of blebs or crystals of quart/; iron 

 oxides, particularly magnetic and titan iferous ; apatite, 

 etc Hence they are commonly distinguishable from the 

 simple rocks by their greater hardness, toughness, and 

 weight Granite, syenite, quartz-porphyry, diabase, 1 

 diorite, trachyte, are examples of compound rocks. 



Many varieties of texture occur among these rocks. 

 The following are among the more important : ( 



FIG. 8.- Piece of porphyritic and cellular lava (showing crystals 

 and steam-holes). 



crystalline ; fine-crystalline ; (cry pto-cryst alii ne, where the 

 crystals are so minute as to appear only under the 

 microscope, might be placed by the field-geologist in 

 the compact series) ; porphyritic having large crystals, 

 usually of felspar, scattered through a compact base 

 (Fig. 8); cellular full of spherical cavities formed by the 

 expansion of imprisoned steam during the outflow of the 

 rock (Fig. 8) ; scoriaceous roughly and irregularly vesicu- 

 lar, like the scoriae of a lava stream, or the "clinkers" 

 from a foundry; amy^daloidal full of almond-shaped con- 

 cretions of calcite, calcedony, zeolites, or other minerals ; 



