PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 457 



Scolescite, thomsonite, lintonite.] 



No. 625B. SCOLESCITE, THOMSONITE(?), LINTONITE, ETC. 



From the same place as No. 625A, but gathered from the beach. 



Meg. Structure and general appearance the same as of No. 625A. 



Mic. A good section made by Marchand, shows two minerals closely mixed. 

 One has sharp, rigid fibres, which pierce the other, running through several individ- 

 uals, forming about one-half of the whole. This is of low double refraction, negative 

 elongation and oblique extinction, and can be safely taken to be scolescite or mesolite. 



The other mineral is so thickly pierced by the fibres of the former that it 

 appears to have a similar fibrous structure, but that is illusory. Still, when the 

 coarse fibration is wanting in any one of these grains, this mineral nevertheless 

 reveals a very fine and dense fibration by reason-of the black bars of a cross which 

 it gives between crossed nicols, which remain on rotation of the stage. These fine 

 fibres, which also give to the grains an imperfect elongation and which are invisible 

 even on lowering the condenser, have a positive elongation. The axial plane is parallel 

 to these fine fibres, and the optic angle is so small that the bisectrix ,, appears like a 

 constant black cross, only being dislocated sufficiently to detect the opening between 

 the hyperbolas. It occurs, hence, that some of the broad longitudinal sections are 

 nearly dark continually, and some show color. The double refraction appears to be 

 but little more than that of quartz; yet, when, in some parts of the slide, the axis 

 n m is perpendicular, the color reaches yellow or yellowish red. This may be due to 

 variation in the thinness of the section. The mineral contrasts greatly with the 

 fibres which pierce it, in its range of coloration and of light which pierces it parallel 

 to n m , and especially in its habit, which is blunt, roundish, spreading, and, as cut in 

 the slide, approximating granular. It might be mistaken for quartz, were it fibrous 

 and not biaxial with n f in the acute angle, or for thomsonite were not its axial plane 

 parallel with its elongation. Some of its forms are very much like the spreading 

 fans of thomsonite. It remains unidentified. (Compare thomsonite found on Isle 

 Royale, Nos. 573 and 575.) 



The slide also shows a little lintonite, and another contains only lintonite. 



Some of the pebbles from the beach are small, hard, fine grained and light green 

 in color. They consist of lintonite. They have w g for acute bisectrix, the optic axes 

 being very nearly together (2 E -about 60), the elongation is positive and negative, 

 and hence, the axial plane is transverse to the elongation, resembling thomsonite in 

 that respect.* The double refraction of lintonite is but little more than that of 

 quartz. For the purpose of making an approximate measure, a slide was prepared 

 on which were ground simultaneously, to the same thinness, four plates of barite, 



* In a discussion of lintonite in the American Geolwjist, vol. xxii, p. 349, the writer inadvertently stated that the axial 

 plane of this mineral iw parallel to the elongation. 



