356 B. K. Emerson— The Deerfield Dyke and Us Minerals. 



Peoducts of decomposition of natkolite. Saponile.—In 

 many cases the natrolite has changed peculiarly into saponite. 

 At first the tips of the needles become opaque and crumbling, 

 and this extends to the whole group, and the whole sinks down 

 into small masses from milk-white to straw-color, which are 

 spread over the surface of the prehnite like a thick layer of 

 dried cream with abundant shrinkage-cracks. Tlie hardness is 

 one or less. It gives up only a little iron with hydrochloric, 

 and is quickly decomposed by hot sulphuric acid. A portion 

 introduced into a drop of water falls asunder much like starch 

 — which it every way resembles — with a series of slight explo- 

 sions which occur at first in rapid succession and continue, 

 growing fainter and farther apart for some little time, until the 

 whole is spread in a broad circle upon the glass. This process 

 exposes minute glassy fragments of prehnite. The flakes after 

 their dispersion present a peculiar appearance under the micro- 

 scope. The first impress I hat of a slide of nearly 

 dried blood corpuscles of uniform size •005--007 mm , the single 

 discs being variously attached to each other. Separate groups 

 of discs often simulate closely in shape the grub of the com- 

 mon May beetle. The scales polarize brightly in bluish tints 

 and arc indistinguishable from kaolin. In many cases a small 

 mass of the mineral seems to have undergone this vermiculite- 

 like reaction in the vein itself, and a delicate bloom is spread 

 in a circle over the prehnite or epidote surrounding a minute 

 hollow cup of the same white or straw-colored material. In 

 one curious case a minute hollow cube with a side broken in, 

 has a tuft of natrolite needles perched in and around it, these 

 latter having minute dodecahedra of fluor strung upon them as 

 described below. The whole is now transformed into or coated 

 with the saponite, and a delicate bloom of the same coats the 

 surrounding epidote. 



It occurs also in hollow pseudomorphs — acute prisms modi- 

 fied by the brachydome and resembling epistilbite. 



The natrolite is succeeded in the prehnite-epidote veins only 

 by calcite, acute scalenohedra enclosing whole tufts of the 

 former mineral, and by fluorite, which is speared upon the del- 

 icate needles of the natrolite in minute transparent colorless 

 octahedra, pierced through a hexahedral axis. From one to 

 four of these are strung upon a single needle, often at the very 

 tip, and in one field of the microscope (X 40), I counted sev- 

 enteen ; and the whole, having as a background the deep 

 green polished faces of the epidote crystals, upon which the 

 group rested, made an object of great beauty. 



The sttlbite-chabazite veins. Stilbitc— This occurs 



