PORPHTEITE. 339 



The Arkansas Dike The long straight dike at the head of the Arkansas presents 

 some remarkable phenomena, which cannot be explained satisfactorily from the data 

 collected in the one short visit made to that area. It is a special matter for regret that 

 no time for further examination could be taken. This dike consists of what are re- 

 garded as two eruptions of the same magma. The older rock is fine-grained and ex- 

 hibits a few small feldspars and biotite leaves as sole recognizable macroscopic con- 

 stituents [130]. The younger rock [129], which cuts irregularly through the former, 

 now on one side, now on the other, or even running along the center, is also very dark 

 and compact in the main, but is sharply distinguished by numerous large quartz 

 crystals and by worn aud well-rounded fragments of slightly pinkish orthoclase. 



A heliotype representation of this curious rock is given in the upper figure, Plate 

 VII, natural size. These orthoclase fragments are all like pebbles, showing no trace 

 of sharp angles. They reach a maximum observed diameter of over 5 cm , and none 

 was noticed of less than l cm . While never glassy, they seem quite fresh, represent 

 but one crystallographic individual each, aud are in no way related to anything seen 

 in other occurrences of the porphyrites. The quartz crystals reach a diameter of over 

 l cm in this rock and are always quite well-defined in crystalline form. Hornblende 

 takes a prominent place beside the biotite in the microscopical constitution. It is a 

 curious fact, commented upon later, that in spite of its large quartz crystals the 

 younger porphyrite has but 59.26 per cent. SiO 2 , while the dark, compact, older rock 

 contains G0.29 per cent. Repeated determinations for both rocks show similar results. 



The origin of these orthoclase pebbles is very problematic. To consider them as 

 earlier secretions of the porphyrite magma is to assume conditions to which no other 

 rocks of the group have been subjected, judging from the total absence of such orthoclase 

 in them. Inclusions of basic microlites would seem to be almost inevitable, if these 

 orthoclase individuals had formed in the midst of the minerals which one must sup- 

 pose to have reached consolidation before them. A thin section of one of these pebble- 

 like fragments shows that the feldspar is common orthoclase aud that it contains 

 inclusions of magnetite, biotite, aud quartz. No zircon, allauite, or apatite was seen. 

 The included quartz grains are small and crowded with fluid inclusions, in many of 

 which the bubble is in active motion. 



From the above considerations it is difficult to reach a conclusion as to the origin 

 of these feldspar masses. The absence of zircon and apatite and the presence of 

 quartz with such numerous fluid inclusions seem to indicate that the mineral is not 

 an earlier secretion of the porphyrite magma. The fact that, while rounded fragments 

 of gneiss and granite are abundant in many dikes in the Archeau, they were not 

 found accompanying these pebbles, would seem to throw doubt upon the accidental 

 nature of the fragments in question. Until a more thorough examination of the occur- 

 rence can be made no satisfactory conclusion seems possible. 



Chemical composition The analyses given below were made by W. P. Hillebrand. 

 Under I is given the composition of the typical horublendic variety, occurring in thin 

 intrusive sheets on the sides of Buckskin and Mosquito gulches. It is Type V of the 

 table above; its macroscopical appearance is shown in Plate VII, lower figure, and its 

 microstructure in Plate XX, Fig. 3. The specimen analyzed came from the Northern 

 Light claim, in lower Buckskin gulch [126]. The rock as a whole is quite fresh, 

 although the few biotite leaves and occasionally a hornblende crystal are more or less 



