82 THE GEOLOGY OP MINNESOTA. 



[Gabbro. 



much and the two periods are not sharply separated. In some places the smaller 

 tabular plagioclases present lath-shaped outlines, and as the pyroxene approaches 

 augite the rock becomes a coarse diabase, with more or less of an ophitic structure. 



Microscopic characters. The feldspar is usually remarkably fresh, and displays 

 brilliant color-bands, when not too thin, according to the widths of the intersected 

 lamellae. "Tested by the Levy-Pumpelly method it gives angles of 13 to 14, and 

 therefore would be classed as labradorite. It contains numerous tubular cavities 

 arranged parallel to the twinning planes; also many glass and other inclusions. The 

 feldspar is somewhat altered and cloudy, some of the sections having suffered greatly. 

 Included in the feldspar is magnetite, chlorite (viridite), quartz and diallage closely 

 approaching augite. The diallage is in places altered to uralite, etc." (Wadsworth.) 



A thin section parallel to the brachypinacoid (010), cut from one of the porphy- 

 ritic crystals, bringing out the cleavage parallel to the base (001) affords an extinction 

 angle of 25 to 27, which, according to the table of Fouque,* indicates that the feld- 

 spar is labradorite-bytownite. 



The twinning striations are more frequently seen on the edges of the tabular 

 crystals (albite law), which exhibit also sometimes the less easy cleavage parallel to 

 the brachypinacoid, than on the sides which are formed by the development of the 

 brachypinacoid. These edges sometimes consist of a wide band without striations, 

 adjoining another which is abundantly striated. Sometimes all the twins which 

 stand in the same direction are narrow, like threads, consisting of the merest films, 

 while their reversed fellows are broad. A basal section may therefore be governed 

 largely, in its direction of easy cleavage, by the broad bands parallel to one of the 

 sets of albite twins, and the narrow bands which are due to the thin albite twins may 

 be inconspicuous or, in small slides, entirely wanting. This apparent absence of 

 banding in basal sections is more likely to occur when the slides are so thin that they 

 do not give colors higher than the grays of the first order in Newton's scale, in which 

 case the lines separating the twins might be mistaken for cleavage. But even then 

 the alternating extinctions are seldom obscured! 



The section examined affords an interesting case of Carlsbad twinning in combi- 

 nation with albite twinning. One Carlsbad is cut parallel to the ba,se, the other being 

 oblique, and the two can be examined by the method of Michel-Levy for the four 

 positions of " eclairement commun." f 



In a section cut at random from No. 1, the greatest equal extinction angle on 

 opposite sides of a twinning macle was found to be 38 on one side and 37 P on the 

 other. This, alone, is not diagnostic for labradorite, for anorthite has the same (see 

 plates VI and VII, Determination des Feldspaths). But since in labradorite this 



'Bulletin tie la ,S'r. </,'/,' FVonffltoi <l- Miin'ruini/if, vol. xvii, p. 428. 

 f Determination dcs /'V/*/.s7;*///*.s, pp. 20-22. 



