Natrolite Description

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Vol. 5]	Louderback.—Benitoite.	355

is always narrow and strongly striated vertically and difficult to get readings on. Its position in the prism zone is definite and its even truncation of the angleleaves no doubt as to its identity. As a constituent form of the crystals g is quite definite and generally present, but its attitude is somewhat variable. It is always more or less curved and generally dull. While in general it lies in the zone of , it appears to be sometimes curved in such a way that its traces with are not parallel, and in such eases the distance between these edges is always greater along the edgeand diminishes outwards in both directions. Where most simply developed it is a narrow band and lies sensibly in the zonesand corresponding to the symbol; but where more broadly developed the lower portion approaches in attitude to Measurements made on the poor reflections of the lower portions of curved faces gave Habit. By far the larger number of neptunite crystals are attached at one end and grow out as comparatively slender prisms six or eight times or even ten times as long as broad. A moderate number show development of faces at both ends of the prism axis, but the habit is always prismatic, and is somewhat similar to that described by Wallenstrom, but in the Greenland prismatic neptunite the orthodome is the most important terminal form, and occurs also in the first found prismatic type of the mineral described by Nordenskiold. In the California neptunite as far as known both ortho- and clino-domes are entirely lacking. The basal plane is always small and brilliant and in outline commonly developed obliquely to the symmetry plane; g is very commonly dull and p frequently so;is generally, frequently andsometimes striated parallel to the edges of the diagonal zone to edges of the corresponding zone,The front pina- coid and the prismare always present and the planeness

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