Wo. 2318. PETROGRAPHY OF DIKE ROCKS OF IDAHO— SHANNON. 479 



mixed character which characterizes many of the dikes may be con- 

 strued to show that when they reached their present position they 

 had already cooled to the extent of partial crystallization and lumps 

 of solidified material were carried forward mixed with more fluid 

 fused magna in a pasty mass yielding solid bodies of more or less 

 agglomeratic texture. 



The age relations of the lamprophyres are rather definitely fixed 

 by the work of Hershey on the age of the Coeur d'Alene batholith in 

 relation to the ores and by the fact that both the ore-bearing veins 

 and the Coeur d'Alene batholith itself are cut by typical lampro- 

 phyric dikes. The whole series of events, tabulated above, from the 

 first underlying intrusion to the injection of the lamprophyric dikes 

 must have occupied a relatively short period of time. That the 

 period of mineralization had not entirely closed before the intrusion 

 of the basic dikes is evidenced by the fact that the dikes are all af- 

 fected by hydrothermal alteration, some of them to an extreme de- 

 gree. 



The lamprophyres occupy narrow dikes rarely more than 10 feet in 

 width and seldom traceable along the strike for more than half a 

 mile. The majority of them strike in a northwesterly direction and 

 dip steeply to the southwest, but exceptions to' this rule are abundant. 

 In appearance the rocks are fine-grained, dark gray, greenish, or 

 black, but lighter-colored, coarser grained varieties occur as described 

 below. They are commonly much weathered in surface outcrop, the 

 majority of the dikes being concealed by soil. In weathering they 

 disintegrate to a friable sand along joints and give rounded cobbles 

 which shell off in concentric layers. The weathered rocks are com- 

 monly greenish to brown from the development of chlorite and 

 limonite from the alteration of the ferromagnesian minerals. The 

 effects of ordinary weathering do not extend any great distance 

 do\vnward, however, and the altered character of the dikes exposed 

 in cuts and mine workings is from the development of sericite and 

 other minerals indicating the action of thermal solutions. 



In the field these rocks may readily be divided into two classes 

 according to the character of the dominant ferromagnesian mineral. 

 The rocks studied may with a few exceptions be called minette or 

 spessartite. The hornblende rocks in the main have plagioclase as 

 the dominant feldspar while most of those which show biotite as 

 the predominant ferromagnesian mineral contain only alkalic feld- 

 spar. Kersantites, or lamprophyres composed of biotite and plagio- 

 clase, are rare, as is the hornblende-orthoclase rock, vogesite. The 

 most abundant varieties are those consisting of greenish-brown horn- 

 blende and twinned lime-soda feldspar. These range from very fine- 

 grained rocks to some which have the texture and appearance of 

 fine-grained hornblende granite. A single specimen of the coarsest 



