THE CHESTER EMERY BED. 145 



reaches a thickness of" 12""". This is at times followed bv (4) (lisaj)ore, 

 which crystallizes around the blades of corundum. 



In some veins the whole series is closed (5) by an abundant develop- 

 ment of margarite; in others by a layer of calcite ((!), up into which the 

 thin, knife-like blades of the corundum project, gTaphic-granite-like, and 

 on etching away the calcite delicate parallel threads of the corundum 

 appear, with blades of corundophilite attached to them or floating freely 

 in the calcite, as well as a beautiful lacework of rutile needles crossing at 

 60° and 120°. It is in these veins that the corundo|)hilite changes into 

 the pale-g'reen amesite of Shejjard. 



Another vein, 1.0"'"' across, shows the following interesting paragenesis 

 beginning with the walls of corundophilite-schist on either side: 



Millimetei'!!. 



(a) A thin, silvery layer of niargarodite in transverse plates 0. 5 



{b) Compact epidote 5. 



(c) Bright flesh-colored plagioclase (oligoclase) 2. 



(d) Mixture of last with fibrous crystalline epidote 7. 



(e) Transparent square plates of diaspore in pockets along the 



central suture .3. 



A third generation of minerals, plainly- of nuich later origin, closes 

 the series. This consists of layers of specular iron (/) in small rosettes of 

 bright scales, upon which aragonite (,(/) occurs in rosettes of long, thick 

 blades, 35-40™" across, in small tufts, and in thick, granular, sugary, white 

 crusts, with some pyi'ite and chalcop}'rite, and finally the whole is often 

 covered with a layer (/;) of small wine-colored I'hombohedra of calcite, f II, 

 and films of malachite. 



It seems to me most probable that the emery-magnetite vein was 

 originall}' a deposit of limonite which was formed by the replacement of 

 limestone, and into whicli, as in the Berkshire County limonites, alumina 

 was carried by infiltrating solutions and deposited as allophane and gibbsite. 

 The subsequent metamorphism of the bed, although it may well have been 

 intimately connected with the extremely violent mechanical forces to which 

 the strata have been subjected, was largely completed before these forces 

 had ceased their activity, as is shown by the jointing and brecciation of 

 the magnetite and emery, and by this metamorphism were formed magnetite 

 and corundum, and, so far as silica sufficed, the very basic corundopliilite 

 (SiO, 24, AlA 25.9, Fe(") 14.S, MgO 22.7, ILO 11.9). The heavy str;itum 



MON XXIX 1(1 



