294 REGINALD A. DALY 



In the Norm classification the rock falls in the domagnesic 

 subrang, wehrlose, in the permiric section, wehrliase, permirlic 

 rang, wehrlase, and section hungariare, of the dofemane order, 

 hungarare. 



According to the accepted Mode classification the rock is an 

 ultra-femic porphyritic olivine gabbro or gabbro porphyrite. The 

 analysis is very similar to the average of three typical wehrlites 

 entered in Part II of Osann's Beitrdge zur chemischen Petrographie. 

 See column 2 of the accompanying Table I. 



Ultra-femic olivine basalt, flow of 1852. — Following the trail 

 from the Volcano House to Mauna Kea, the writer crossed the 

 lava which, in 1852, flowed out on the Hilo slope of Mauna Loa. 

 The trail traverses this lava at the 6,100-foot contour (see locality 

 "2" in Fig. 1), where the flow is about 1.5 kilometers in width. 

 The lava is of the aa or block type and much work with sledge 

 hammers was necessary to make a trail passable even for the hardy 

 pack-animals of the island. The broken rock is, of course, quite 

 fresh, and its numerous olivine phenocrysts, of unusually great 

 size and of beautiful color and brilliance, made a remarkable effect 

 for the eye as one walked or rode over the lava. The development 

 of phenocrystic olivine is greater in this flow than in any other 

 seen by the writer during several hundred miles of travel in Hawaii. 



The hand-specimens of the lava have a dark-gray, lithoidal 

 ground-mass, in which the abundant, bright yellowish-green 

 olivines are conspicuously set. As usual with the aa type of lava, 

 the gas pores are large and irregularly distributed through the 

 rock. They were elongated and flattened during the flow of the 

 stiffening lava and the longer diameters of the pores reach three 

 or more centimeters in length. The idiomorphism of the olivine 

 is often manifest to the unaided eye, and is still more evident 

 under the microscope. The individual crystals are often more 

 than one centimeter in diameter. No other mineral is pheno- 

 crystic. 



The microscope shows a rather surprising contrast in the grain 

 of the ground-mass, which is of diabasic structure, with thin 

 tables of plagioclase, seldom over o . 1 mm. in length, separated by 

 augite granules of even smaller diameters. Magnetite and prob- 



