IGNEOUS FORMATIONS. 307 



discussion is necessary. The unusual ratio of the alkalis is the important 

 fact, pointing to the probable existence of orthoclase in the rock, which 

 has already been asserted from microscopical examination. 



TABLE MOUNTAIN BASALT; EARLIEST LAVAS. 



occurrence. — On North Table Mountain, at a horizon which is about 

 100 feet below the capping sheer, there are exposed a number of compar- 

 atively small masses of basalt. They possess the typical features of surface 

 flows, as minutely described above, and the exposures must be considered 

 as sections of lava streams from the first eruption of basalt in this region, 

 the quantity erupted being insufficient to form a continuous sheet as in the 

 later outbreaks. These streams are of a maximum thickness of 50 feet, and 

 they exhibit the same structure as the larger one above them on the 

 mountain. 



conditions of consolidation. — As these streams were at once covered after erup- 

 tion by sandy material like that upon which they rest, it is most natural to 

 suppose them to have been poured out into the sea. for there, are no visible 

 reasons for supposing the eruption to have occurred during an interval in 

 which no sediments were laid down. The smaller mass of these streams 

 makes the conclusion necessary that their consolidation was somewhat 

 more quickly completed than in the case of the larger masses described. 

 As the first outbreak in a series from a common source, it is not strange 

 that the bulk analysis shows a slightly different magma. 



Description. — The basalt of these early streams is a dense, black rock 

 characterized bv many augite and olivine phenocrysts, lying in a denser, 

 darker groundmass than is shown by any part of the upper sheets. 

 Phenocrvsts of plagioclase are present, but they are megascopically very 

 inconspicuous. 



The microscope shows the augite to have a pleochroism from yel- 

 lowish to greenish shades, and the crystals are both larger and better 

 formed than in any of the rocks previously described. Olivine, too, is 

 developed in larger crystals than common, and is very fresh as a rule. The 

 tew plagioclase phenocrysts are filled with a cloud of minute inclusions 

 excepting in a (dear outer zone. The rounded forms of the parts rich in 

 inclusions suggest a period of resorption. 



