No. 2318. PETROGRAPHY OF DIKE ROCKS OP IDAHO— SHANNON. 491 



main mass of the rock. The feldspar phenocrysts are zoned and 

 show albite and carlsbad twinning. They range in composition from 

 oligoclase-albite, Ab^a Ang, at the peripheries to bytownite-anorthite, 

 Abi7 Angg, at the cores. Quartz and micropegmatite are common 

 in the interstices of this rock. 



iSpessartite, Eldorado claim. — At the portal of the tunnel of the 

 Eldorado claim, a prospect opening on a small barren vein in the 

 Prichard formation on the north side of the river, about a mile east 

 of Kellogg, a small amount of quarrying has been done on a broad 

 exposure of lamprophyric rock. This may be in the form of a neck 

 or stock as it can not be traced in any direction from this exposure. 

 There is some evidence that narrower dikes of ordinary more basic 

 lamprophyre may radiate outward from this mass and the quartz- 

 pyrite vein developed in the tunnel terminates at the igneous rock in 

 a suggestive manner. In the hand specimen the rock is fairly 

 ("oarse granular and consists of equal parts of white to pale flesh 

 colored feldspar and greenish hornblende. Occasional large poikilitic 

 biotites occur and small veinlets of pegmatitic material were seen. 

 Under the microscope the rock is a coarse aggregate of hornblende 

 and feldspar. The larger feldspars which are badly sericitized in- 

 ternally are plagioclase. Smaller grains appear to be orthoclase. 

 The hornblende is of the usual type. Large chlorite patches dotted 

 with iron ore and surrounded by hornblende rims appear to repre- 

 sent original augite. Dots of iron ore and a few long prismatic apa- 

 tites occur. (See pi. 38 B.) 



Spessartite, Lomhardy claim. — The first 70 feet from the portal 

 of the lower tunnel of the Lombardy claim in Italian Gulch north of 

 Kellogg are in a peculiarly equidimensional mass of lamprophyric 

 rock precisely like that last described on the Eldorado claim. There 

 is evidence that one or more dikes of finer grain and more mafic com- 

 position may extend outward from this mass. The Lombardy vein, 

 can-ying some lead and great masses of cupriferous pyrite and 

 pyrrhotite, also extends outward from this igneous mass. Megascop- 

 ically the rock of this occurrence is indistinguishable from that of 

 the Eldorado claim and the microscopic character is similar. The 

 feldspar, which is much altered, appears to be a rather basic plagio- 

 clase. The hornblendes which are sharply euhedral show some in- 

 cipient alteration to green chlorite but are for the most part fresh 

 with well-developed cleavage. They are as usual pleochroic in tones 

 of light to dark greenish brown but have borders of green hornblende. 

 The, rock contains large areas of calcite. Associated with these 

 are patches of serpentine with hornblende rims which are probably 

 altered olivines. Biotite occurs rarely and in the form of small 

 tablets. Scattered stout well-formed prisms of apatite and grains 

 of iron ore occur. Quartz is absent. 



