February 28, 1913] 



SCIENCE 



339 



alaskite porphyry, but in all other directions 

 it disappears under the detrital plains of the 

 desert. The area exposed — about ten square 

 miles — probably represents only a remnant of 

 a more extensive intrusion. The alaskite por- 

 phyry, the intrusion next in age, is a rock with 

 a fine felsitic ground mass, carrying pheno- 

 crysts of quartz and orthoclase seldom over a 

 millimeter in diameter. It resembles the 

 alaskite in composition, though showing more 

 variation in kind of feldspar. An intrusive 

 contact between the porphyry and the coarse 

 alaskite was found, but the other limits of the 

 porphyry do not fall within the region studied 

 It was examined over an area of about three 

 square miles. The biotite granite is a holo- 

 crystalline rock with an average grain of a 

 quarter of an inch. It is composed of ortho- 

 clase, a little plagioclase, quartz and biotite, 

 this last mineral sometimes forming pheno- 

 crysts. The relation of this granite to the 

 other rocks is the most interesting petro- 

 logic feature of the district. It is found 

 only in the alaskite porphyry, and occurs in 

 three forms: (1) As irregular stocks about 

 fifteen hundred feet in diameter, (2) as small 

 bunches or lenses sometimes only a few feet 

 in dimension, and (3) as well-defined dikes 

 fifteen to twenty feet wide along the contact 

 of the alaskite porphyry and the limestone 

 blocks. In the first two cases the texture is 

 holocrystalline and strikingly coarse even at 

 the contacts. In the third case the rock is a 

 granite porphyry — phenocrysts of quartz, 

 feldspar and biotite in a glassy ground mass. 



The fourth rock is more basic than the 

 others, and may be a dacite porphyry, but the 

 large amount of irresolvable ground mass 

 makes its classification uncertain without 

 chemical analyses. 



The explanation ofi^ered for the above facts 

 is as follows : The original rock magma was 

 an acidic granite, which split into two parts, 

 one rich in biotite, the other practically mica- 

 free. The more acidic portion was intruded 

 first, forming the alaskite and the alaskite 

 porphyry. When the latter rock was only 

 partly cooled, the biotite-bearing portion was 

 intruded, working its way into the still pasty 



alaskite porphyry to form lenses, tongues and 

 other irregularly defined masses. Along the 

 borders of the alaskite porphyry cooling had 

 gone further, clean fissures had been formed, 

 and in such places well-defined dikes of gran- 

 ite porphyry resulted. This explains the inti- 

 mate relationship between the granite and the 

 alaskite porphyry, its various irregular shapes, 

 and the coarse texture in some instances and 

 the fine texture in others. There is a possibil- 

 ity that the quartz porphyry, which contains 

 much biotite, is a still later intrusion of the 

 biotitic phase of the same magma, but distinct 

 evidence on this point is lacking. The rela- 

 tion between the quartz porphyry and the 

 alaskites is not as close as between the granite 

 and the alaskites. The quartz porphyry seems 

 to belong to a distinctly later period of in- 

 trusion, while the granites and alaskites are 

 of very nearly the same age. 



Although aware of the objections urged 

 against similar hypotheses, I am inclined to 

 attribute the splitting of the magma into two 

 portions to fractional crystallization and the 

 sinking of the heavier biotite crystals. 

 Whether or not this last point is well taken 

 can not affect the conclusion that the granite 

 is a later differentiation phase of the magma 

 from which the alaskites came. 



The origin of the granite has important 

 bearing upon the genesis of the ore-deposits. 

 These ores are contact metamorphic copper de- 

 posits in the limestones at the contact with 

 alaskite porphyry. They are attributed to 

 magmatic waters given off by the intrusive, 

 in accord with the conclusions reached in 

 similar districts by Kemp, Lindgren and 

 others. Eut it is noteworthy that the richest 

 ore is found in the neighborhood of masses of 

 granite. Now if this granite is the final prod- 

 uct of differentiation of the alaskite magma, 

 it is very probable that it would bring with it 

 increased quantities of magmatic water. The 

 granite and the granite porphyry then partake 

 somewhat of the nature of a pegmatite in that 

 they represent fi;ial products of local magma 

 splitting; they differ from pegmatites in tex- 

 ture, and from contemporaneous veins in gen- 

 eral in their greater extent. The above facts 



