204 BASALT. 



Only a few additions will be made to Toriiebolim's description from Dr. Smith's 

 sections. The Ovifak sections have the usual structure of a dialjase or dolerite ; 

 divero-ent crystals of plagioclase lying in and dissecting the irregular masses of pale 

 brown augite, iron ores, olivine, etc., which form the interstitial material between 

 the feldspars. The minerals are the same in general characters as those described 

 by Tornebohm. In some cases the glass shows the globulitic structure common in 

 basaltic glass. 



This basalt is more or less altered in the different sections, presenting many of 

 the characters of a diabase, and the green and brown silicates, replacing glass, olivine, 

 iron, etc. 



Much graphite occurs in scaly aggregations of a black color with a lustrous reflec- 

 tion in reflected liglit, and associated with a brown and violet-red mineral which has 

 been referred by Tornebohm to spinel ; but in the section examined by myself, part 

 has been found not to be isotropic, and has been considered by Dr. Smith to be corundum 

 (1. c. pp. 484-486). 



The reader is further referred to the before-mentioned full and excellent descrip- 

 tion of Tornebohm for a more extended study of this basaltic rock; as well as to the 

 writings of Tschermak.* 



One section in the Lithological Collection shows a grayish-white groundmass filled 

 by rounded grayish-black masses. These dark spots are seen under the auicroscope 

 to be composed of plagioclastic feldspars, fllled with an irregular network of granules 

 and masses of magnetic and native iron, the whole closely resembling the structure 

 of some portions of the Estherville meteorite (Plate III. fig. 6). The interstitial 

 portions between the rounded feldspathic-iron masses are filled by tlie normal basalt, 

 composed of ledge-formed plagioclase crystals, cutting a mass of yellowish-gray augite 

 grains, violet-brown globulitic glass, magnetite, and the viriditic products. 



In some parts of the section tlie large plagioclase crystals are free from the iron, 

 and contain glass inclusions and minute pores. A little olivine, some large augites, 

 and yellowish-brown hisingerite (?) was seen in this portion of tlie section which has 

 a doleritic or diabasic structure, while other portions have that belonging distinctively 

 to the fine-grained basalts. 



Another section from the same hand-specimen shows in part of its mass the same 

 basaltic structure as the preceding, of plagioclase, augite, magnetite, base, and secondary 

 materials ; but the remaining portion is a coarsely crystallized mass of olivine, plagioclase, 

 augite, iron, and magnetite. The olivines are in irregularly rounded grains, traversed by 

 fissures They are sometimes clear, and at others stained yellowish, and are altered along 

 the fissures to a yellowish and brownish serpentine. The augites are pale-yellowish, and 

 with the olivines contain bubble-bearing glass inclusions, iron, magnetite, etc. The usual 

 secondary products occur to some extent. Some of Professor Smith's sections have parts 

 similar to these last two sections, except the coarsely crystalline olivine-bearing portion ; 

 but his are more altered, and contain a larger araoimt of secondary products. 



Two of Dr. Smith's sections from Pfaff-Oberg are seen to be composed of lath-shaped, 

 divergent plagioclase crystals, lying in a granular groundmass of augite, olivine, etc., with 

 various secondary products. In one section is a large grain of iron, of an irregular cel- 

 lular structure, and holding in its cells pyrrhotite, olivine, feldspar, etc. 



The preceding descriptions show that the coarse and fine crystalline structure 



* Miu. Mitth., 1874, pp. 171-174. 



