IGNEOUS FORMATIONS. 299 



heated from the passage of the lava, and the consolidation thus took place 

 under considerable pressure, and when- the cooling must have Keen rery 

 slow and uniform for all parts "t' the dike now visible. 



Description. — The basalt < »f the Valmont dike is a dark greenish-gray 

 mck with a porphyritic structure, occasioned chiefly by the prominence 

 of black augite prisms, which reach a length of 1 cm. by a breadth of 0.5 

 cm. Close examination reveals some augites, and indistinct plagioclase 

 tablets. A hand lens shows the mass of the rock to consist of plagioclase 

 tablets quite irregularly arranged, and the dark color is partly due to 

 minute black and green specks disseminated abundantly through the whole. 

 Colorless olivine crystals appear to have a brilliant black, metallic luster 

 through total reflection from curved fissure planes. 



Microscopical investigation shows the rock to consist chiefly of plagio- 

 clase, augite, and olivine, while orthoclase and biotite are also important 

 constituents, and magnetite and apatite play the usual accessory roles. 



The augite has a dull greenish-gray color and its phenocrysts possess 

 the usual characteristics as to form and inclusions. The minute green 

 grains visible with a hand lens are also augite, of the same properties as 

 the large crystals. They are irregular in shape, and vary between 0.05 

 nun. and 0.20 mm. in diameter. They seem to be of a later generation 

 than the large crystals, but it also appears that in this second period of 

 growth the phenocrysts received local additions, causing the outlines of the 

 most perfect prisms to become microscopically irregular. 



Olivine is developed in normal form and abundance. It is quite 

 fresh as a rule, but shows very beautifully in some crystals the process of 

 serpentinization, the product being golden-yellow or sometimes greenish 

 fibers normal to the fissures. There are many minute olivine grains 

 corresponding in size to those of augite, but it is not evident that these are 

 of a second generation, for there is an almost perfect gradation in size 

 between these particles and the large crystals. 



The main mass of the rock is feldspathic and its structure is visible 

 only in polarized light. There are numerous narrow pinacoidal tablets of 

 plagioclase 1 mm. to 3 mm. lone-, with tine lamina' twinned accordingto the 

 albite law, and with further twinning after the Carlsbad and pericline laws 



