362 F L. Ransome—Lava Flows of California. 



are plainly of foreign origin, having been caught up by the 

 lava as it flowed over the underlying andesitic breccias. The 

 accessory minerals are apatite, magnetite, green hornblende, 

 and in one or two cases a few crystals of hypersthene. 



The Dardanelle flow is composed of augite-latite almost 

 identical in character with that of the Table Mountain flow. 

 It is, however, more compact in texture, and the pheno- 

 crysts are smaller. Although characterized by columnar struc- 

 ture, the columns are neither so large nor so regular as those of 

 the earlier flow. Microscopically it differs from the latter in 

 containing somewhat less olivine, and in having a slightly 

 larger proportion of glass. An occasional zircon can be noted 

 among the accessory minerals. 



In view of the amount of potash shown in all of the chem- 

 ical analyses of the latite, and the absence of what are 

 commonly regarded as potash-bearing minerals, considerable 

 interest attaches to the residual glass of these rocks. Even in 

 the Dardanelle flow the proportion of glass is not large, and it 

 is difficult to find parts of the section where the glass can be 

 studied without any overlapping of the numerous laths of 

 labradorite. It is moreover rendered turbid by abundant 

 globulites, which were proved to be in large part magnetite, in 

 a very fine state of subdivision. A portion of the powdered 

 groundmass was passed through a number 120 sieve, the dust 

 taken out through a piece of linen, and the remaining powder 

 well washed. After being thoroughly shaken up in Thoulet 

 solution, about half the powder still floated upon dilution to 

 sp. gr. 2-626. Upon subsequent dilution to sp. gr. 2485, this 

 powder fell, and was readily separable with a weak magnet 

 into two portions, the non-magnetic portion consisting of frag- 

 ments of more or less impure feldspar, while the darker mag- 

 netic powder consisted of globulitic glass with included frag- 

 ments of labradorite laths. The glass itself has thus a specific 

 gravity much below 2'626. Its index of refraction is lower 

 than that of the balsam (1*540). Grains as pure as possible, 

 picked out under the microscope, afforded abundant crystals of 

 potassium fluosilicate when treated with hydrofluosilicic acid. 



Chemical composition of the Sierra Nevada latites. — In the 

 accompanying table on page 363, the chemical analyses of the 

 latites from all three flows are brought together for com- 

 parison. In spite of a certain range in the silica percentages, 

 they are clearly closely related members of a series, not far 

 from the andesites in general composition, but slightly higher 

 in silica, and distinctly so in alkalies, the potash, when reck- 

 oned in percentages, being always slightly in excess of the 

 soda. When, however, molecular proportions are considered, 

 it is more accurate to say that potash and soda are present in 

 nearly equal amounts. As would be expected from the study 



