HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 579 



fectly developed porphyritic plagioclases with or without hornblende or 

 black mica. At times, as in the well known " porfido rosso antico," 

 or antique porphyries of Egypt, the groundmass is micro-crystalline, 

 forming thus connecting links between the true diorites and diorite 

 porphyrites. Indeed the rocks of the group may be said to bear the 

 same relation to the diorites in the plagioclase series as do the quartz 

 porphyries to the granites in the orthoclase series, or better yet, they 

 maybe compared with the hornblende andesites, of which they are ap- 

 parently the Palaeozoic equivalents. 



Colors. — The prevailing colors are dark brown, gray, or greenish. 



Classification. — According to the character of prevailing accessory 

 mineral we have hornblende porphyrite, or diorite porphyrite, as it is 

 sometimes called, and mica porphyrite. When, as is frequently the 

 case, neither of the above minerals are developed in recognizable quan- 

 tities, the rock is designated as simply porphyrite. The porphyrites are 

 widespread rocks, very characteristic of the later Paheozoic formations, 

 occurring as contemporaneous lava flows, intrusive sheets, dikes and 

 bosses. The more important localities and varieties exhibited are given 

 in the following list : 



Elk Mountains, Colorado, head of Willow Greek, 39197 ; Mosquito Gulch, Park 

 County, Colorado, 68976 and 68951 ; north slope Bartlett Mountains, Summit 

 County, Colorado, 68972 ; Cedar Creek, Madison County, Montana, 72866 and 

 72880 ; Jefferson County, Montana, 73170 ; near Liberty ville, New Jersey (mica por- 

 phyrite), 72830; Nantasket, Massachusetts, 38514; Terra del Fuego, South America, 

 1880 ; North Berwick, Law, England, 36386 ; Loch Fine, Argyleshire, Scotland, 

 70380 and 70381 ; Lake Lugano, 36622 and 36616, 36617 and 36618 ; Vallee du Lys, 

 Pyrenees, 36375; Quenast, Belgium, 36377; Falkenstein, in the Fichtelgebirgo, 

 Bavaria, 36376 ; Hochwald and Gottesberg, Silesia, 36389 and 36390 ; Postchap- 

 pel, Saxony, 36387 ; Gienberg, Theodorshall, Munster, Wertenborg, Gonnes- 

 weiler, Wolfstein, and Namborn (epidiorite porphyrite), Nahe, Prussia, 36379, 

 36380, 36381, 38384, 36388, 70242, 70185, 70205 ; Ilfeld, Hartz Mountains, 36385. 



7. The andesites. 



Andesite. The name was first used by L. V. Buch in 1835, to desig- 

 nate a type of volcanic rocks found in the Andes Mountains, South 

 America. 



Mineral composition. — The essential constituents are soda-lime feld- 

 spar, together with black mica, hornblende, augite, or a rhombic pyrox- 

 ene, and in smaller, usually microscopic proportions, magnetite, ilmen- 

 ite, hematite, and apatite. Common accessories are olivine, sphene, 

 garnets, quartz, tridymite, orthite, pyrite, and sanidiu. 



Chemical composition. — The composition of the andesites varies very 

 considerably, the quartz-bearing members naturally showing a much 

 higher percentage of silica. The following table shows the composition 

 of a few typical forms : 



