No. 2318. PETROGRAPHY OF DIKE ROCKS OF IDAHO— SHANNON. 493 



Peculiar interest attaches to a rock which forms a very small 

 branch from the Standard dike into the quartzite of the footwall on 

 the 1,200-foot level of this mine. In the hand specimen the rock is 

 seen to consist of small, greenish grains in an aphanitic black base, 

 the whole looking like an ordinary basalt. It is clearly a result of 

 sudden cooling on the magma which composes the larger dike. 



In thin section the rock is seen to have originally consisted of 

 abundant large well-crystallized phenocrysts of olivine and smaller 

 prisms of augite, in a matrix consisting of glass completely filled with 

 minute prismoid laths of augite. Scattered rather large grains of 

 iron ore also occur. The olivine is now entirely changed to aggre- 

 gates of fibrous talc, no vestige of the original substance remaining. 

 The original cracks, typical of olivine, are now preserved by dotted 

 lines of iron ore. The augite phenocrysts are almost as completely 

 altered to chlorite with some secondary silica. Some of the original 

 pyroxene still unaltered was identified as near diopside by its ex- 

 tinction angle. Many of the large olivines have been shattered and 

 the parts slightly separated from each other, the intervening space 

 being filled by the material of the groundmass. The minute lath- 

 like microlites of augite were identified by their rather high index 

 of refraction, moderately high birefringence, extinction, and square 

 cross section. They show none of the alteration which marks the 

 large augite phenocrysts. 



This rock apparently indicates that the magma, which upon crys- 

 tallization in the larger dike gave a typical spessartite, when in- 

 truded contained abundant crystals of olivine and augite. With 

 slow cooling these phenocrysts, by reaction with the magma, were 

 changed to hornblende as evidenced by the talcoid pseudomorphs 

 surrounded by hornblende rims in the main dike on the 1,200-foot 

 level. On the 2,000-foot level at some 800 feet greater depth, where 

 cooling presumably was slower, the olivines were more completely 

 resorbed and no vestige of them now remains. Whether this change 

 is due to absorption of silica from the walls of the fissure by the 

 magma in its upward progress is not clear, but it forms an interest- 

 ing hypothesis. 



Spessartite, Reeder Gulch. — This rock, one of the few of those 

 described which came from the Murray region, forms a very long 

 dike exposed in the mouth of Reeder Gulch by the side of the 

 Golden Chest mill. Megascopically it is a very light-colored rock 

 of nonvitreous chalky appearance and fine grain, but with widely 

 spaced phenocrysts of very black biotite. Under the microscope it 

 exhibits tw^o materials of very different texture but of the same 

 inineralogical composition. The contact between these is sharp, one 

 being fine and the other coarse grained, and both consisting of horn- 

 blende prisms and laths of plagioclase. The coarser portion con- 

 tains large aggregates of radiating prismatic epidote, especially 



