PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 409 



Lintonite, mesolite, okenite. Diabase.] 



or only partly of thomsonite. These very fine bright fibres are distributed through 

 it. They are always negative in elongation. It has to be assumed that they are 

 associated with about an equal number that are cut so as to present n s perpendicular, 

 but these cannot be distinguished. That the whole band is not of thomsonite is 

 probable from the fact that, on tracing the fibres toward their converging point, while 

 the bright fibres disappear, there are some fibres which are cut transverse to their 



4 



elongation. Their sections are quadrilateral and their color of double refraction is 

 low even lower than that of the bright needles of thomsonite. They must be 

 therefore of mesolite, for a similar section of thomsonite would show, as already 

 stated, a color either red, blue or green. 



It is hence allowable to infer that mesolite and thomsonite are closely inter- 

 grown throughout the band in which there are occasionally the bright needles which 

 have always negative elongation. 



In none of the sections is there a band made up wholly of thomsonite in the 

 midst of mesolite bands. 



That these bright needles are of thomsonite and not of mesolite is evinced by 

 the fact that they occur sometimes in the midst of a coarser mesh of mesolite fibres 

 which of themselves show slight difference of light, indicating that they lie some- 

 times in a position to show ,, and sometimes n f perpendicular. 



A green pebble having this number, collected as lintonite, proves to be not 

 homogeneous, but consists largely of fragments or imperfect crystals of an amphibole 

 mingled with zoisite and some sphene, with other indeterminate substances. N. H. w. 



No. 535B. LINTONITE, MESOLITE, OKENITE(?) 



An unfavorably thick section shows the first two of these zeolites distinctly, but 

 in the same mass as the mesolite the mesolite fibres which are negative in elon- 

 gation are replaced in the direction toward the apex of the fibration by an entirely 

 different mineral which, with much coarser fibres, parallel extinction and low double 

 refraction, thus resembling scolescite, has a positive elongation, a combination of 

 characters which would allow of its being okenite. Without chemical test, however, 

 this is only a provisional determination. 



One section. N. H. w. 



No. 536. DIABASE. (Gabbroid.) 



From the basaltic columns at Grand Marais (see No. 199). 

 Ref. Annual Report, x, page 43. 



Meg. A brownish rock of medium grain, standing in vertical columns, in a 

 manner similar to the rock at Little Marais, forming the barrier that encloses the 

 harbor of Grand Marais. 



