480 PROCEEDINGS OF THE NATIONAL MUSEUM. "oi.. 57. 



forms might well be classed as a diorite, yet there is every grada- 

 tion between the coarse forms on the one hand and very fine-grained 

 mafic rocks of typical Lamprophyric texture and mode of occurrence 

 on the other. Such rocks to distinguish them from ordinary diorites 

 must be called spessartites (although this name is objectionable, as 

 it has been given also to a mineral of the garnet group). The re- 

 lationship of the spessartites to the minettes is not clear. They are 

 distinct from each other in composition — so much so in fact as to 

 suggest that they were derived from the parent magma at slightly 

 different periods. No data on the relative age relations of the two 

 types were obtained in the field. There seems to be some evidence, 

 however, showing that a given dike may pass from one rock into the 

 other along its strike. Thus the large spessartite dike at Bailey's 

 Pond apparently continues across the valley, yet on the north side 

 of the river it consists of minette. The same thing occurs at Elk 

 Creek where a typical minette is exposed on the soiith side of the 

 valley yet where what seems to be the same dike reappears on the 

 north side of the river, scarcely over a hundred yards distant, it is 

 a much decomposed spessartite consisting almost entirely of horn- 

 blende with neither biotite nor orthoclase. 



The lamprophyres are closely associated with the veins of the 

 sections in which they occur. Around Kellogg almost every one of 

 the large nonproductive pyritic quartz veins has its dike of lam- 

 prophyric rock within a few yards of the vein itself. Such relation- 

 ships are well exhibited in the Lombardy, Teddy, Tillicum, Eldo- 

 rado, Enterprise, Evolution, and other prospects near Kellogg and 

 a similar relation exists in many of the lead mines of the Canyon 

 Creek and Mullan areas as in the Hecla, Marsh, Moonlight, Morn- 

 ing, Rex, Helena-Frisco, Success, Standard, and other mines. This 

 juxtaposition of dikes and veins argues in favor of the conclusion 

 that they have a common source and also shows that at the time the 

 lamprophyres were intruded, in contrast with the enormous abun- 

 dance of later fissures, planes of weakness were few in number and 

 igneous injections were constrained to occupy the same general chan- 

 nels followed by the mineralizing solutions. 



DETAILED DESCRIPTION OF THE L.IMPROPHYRBS. 



I. BIOTITE TYPE. 



Minette, Hecla Mine. — The dike which occupies the same fissure, 

 but is younger than the Hecla vein (see fig. 1), has been described 

 by Ransome, as being chiefly composed of hornblende. Sections 

 were not examined by the writer, and the first specimen which was 

 studied was found on the Hecla dump and does not agree with 

 Ransome's description. Specimens from various parts of the mine 



