226 GEOLOGY OF THE EUllEKA DISTRIOT. 



of a coarse grained granite composed of pellucid quartz, dark brown biotite, 

 and orthoclase in Carlsbad twins, with varying proportions of plagioclase 

 and strongly pleochroic hornblende. It everywhere weathers in rounded 

 masses with rough surfaces, disintegrating like many varieties of granite. 

 It is, however, only a limited portion of the dike which possesses anything 

 like a granitic structure, the greater part of the main dike and all the 

 secondary branches having a decidedly porphyritic structure. All through 

 the central part of the dike the rock is formed of large ill-defined crystals, 

 porphyritically imbedded in a groundmass of the same composition, which 

 under the microscope is seen to possess a micro-granitic structure. This 

 porphyritic structm-e can be traced for miles along all the dikes parallel with 

 their com-se. From the normal type in the central portion of the dike 

 toward the outer limestone wall there is a gradual and at first an almost 

 uuperceptible transition to the finer grained rock, with more and more of 

 the porphyritic and less of the granitic stnicture. Across these transitional 

 rocks the mineral components remain the same, the difterences consisting in 

 the size of the grains and their relative proportions and structural relations. 



All thin sections show the rock to be entirely crystalline without 

 isotropic g'lass. At a distance of from 20 to 30 feetfrom the limestone, vary- 

 ing with the width and position of the dike,the rock shows a marked jior- 

 phyritic habit, though the larger crystals are still in excess. In the nar- 

 rower dikes the transitions are not so well shown as the coarser portions and 

 are less characteristically developed and the changes far less gradual. In 

 the latter the quartz is very abundant and frequently occurs in well devel- 

 oped dihexahedrons, in strong contrast to the quartz, with irregular outlines, 

 as seen in the granitic structure. From here to the limestone contact the 

 change is more rapid, the larger crystals becoming less and less abundant, 

 being replaced by more and more micro-crystalline goundraass. 



Nearly everywhere along the immediate line of contact the rock pre- 

 sents the habit of a quartz-porphyry made up of a crystalline groundmass, 

 with well developed crystals of quartz and orthoclase, and still accompanied 

 by some plagioclase. Both mica and hornblende usually fall off in amount 

 toward the edge of the dike, in the more ijorphyritic rocks, the hornblende 

 being present only in the granitic types, and the first to disappear with 



