170 THE GEOLOGY OP MINNESOTA. 



[Scoria. Laumontite. 



No. 910. SCORIA (with prehnite-}. 



From a vesicular mass in No. 91. This scoria is three feet in diameter, and forms the centre of a larger 

 "concretion" at least six feet through. 

 Ref. Annual Report, ix, page 25. 



Meg. The scoria itself is quite vesicular, and the framework has become rotted 

 to a lilac-colored kaolin. The mass is held together by the growth of a hard zeolitic 

 mineral resembling prehnite, which permeates the whole. This mineral fills the 

 smaller vesicular cavities and fines the larger geodic spaces with a fine, roughened, 

 botryoidal coating. Fractured surfaces show a glistening cleavage, and usually a 

 coarse radiated crystallization which stands perpendicular to the cavity walls. The 

 color is gray, and the hardness is about 6. 



Mic. The double refraction of this mineral is high. The areas of each color 

 are triangular, or sub-triangular, cornering together at the centre of the amyg- 

 dules, thus constituting nearly circular spaces brilliantly colored and variegated. 

 The elongation is negative, and the extinction is parallel, as those terms are used by 

 Lacroix.* The bisectrix is , and the angle 2 E is quite small, not exceeding 10. 

 In HC1 it forms no jelly. These characters sufficiently prove the mineral to be 

 prehnite. 



One section. 



Age. Cabotian. N. H. w. 



No. 92. LAUMONTITE. 



Half way between Knife river and Agate bay. Laumontitic rock and amygdules, with calcite. 

 Ref. Annual Report, ix, pages 25, 26. 



Meg. The laumontite is white, with a pinkish tinge, very friable, arranged 

 radially. It fills very small amygdaloidal cavities, and constitutes masses several 

 inches across. 



Mic. It is made up of fine fibres which darken between crossed nicols at an angle 

 with the elongation which varies from 3^- to 29. The trial is made by placing some of 

 the powdered mineral on the slide (the grains are more visible and more conven- 

 iently manipulated when mounted in balsam and covered with a thin glass) and 

 measuring the angles between extinction and the straight elongated sides of the 

 larger fibres. The smallest fibres have extinction nearest to parallelism with the 

 elongation. The largest flat grains, which may be taken to be parallel with the 

 easiest cleavage, have this extinction angle as large as 29, varying from 23. The 

 largest plates, tested in convergent light, show the optic normal in the interference 

 figure, which shows that the plane of the optic axes is in the plane of the easy cleav- 

 age. With hydrychloric acid a gelatinous silica is at once formed. These charac- 

 ters cannot coincide in any fibrous zeolite except in laumontitefi which is monoclinic. 



*Bulletin de la Soetiti de Miniralugie da France, vol. viii, p. 322. 



tA. LACROIX. Sur le diagnostique des zeolithes en 1'absence de formes crystallines dSterminables. Bulletin de la Socittt 

 de Mineralogie de France, Tome viii, pp. 321-367, 1885. 



