THE CHESTER EMERY BED. 129 



4. DlASI'OItE. 



This is hyclrated alumina, perhaps the most strictly characteristic accompani- 

 ment of emery in the Grecian Archipelago and Turkey. It occurs at Chester precisely 

 as in those regions, viz, in needle-shaped crystals and bladed masses, chiefly upon 

 the cross joints of the emery blocks, though sometimes embedded (in compressed round 

 masses) quite within its substance. It is generally colorless, though sometimes of a 

 pinkish or violet tint. Perfect crystals of the usual form are not wanting where the 

 usual open spaces exist in the masses. Crystals also of corundophilite and very rarely 

 of brookite are found embedded in it. 



5. MARfiAIUTE. 



Scarcely less characteristic of emery, and also of corundum, is the present mineral, 

 a species whose general aspect suggests that of mica, from which it differs in possess- 

 ing a greater hardness and a lower dose of silica with a corresponding increase of 

 alumina added to an almost total absence of either of the alkalis.' The margarite 

 presents itself frequently and with a richness of crystallization and color nowhere else 

 known. It is always in near ijroximity to the purest masses of emery — sometimes 

 traversing it in veins, at others coating, more or less perfectly, large and small rounded 

 masses of it with layers an inch or more in thickness. The lamiuie of the margarite 

 are arranged transversely in respect to the direction of the seams, i. e., they stand at 

 right angles to the walls of the veins. Sometimes an open space exists in the middle 

 of the margarite seam, when the mineral exhibits very rarely regularly terminated 

 crystals with which also crystals of corundophilite are associated. Emery grains are 

 likewise to be detected everywhere among the margarite. Its color is almost univer- 

 sally of a pinkish tint. In a few instances, however, where it occurs in detached 

 scales mixed up with a yellowish epidote in the massive emery, it assumes a grayish 

 color and might be mistaken for ordinary mica, a species which I have nowhere 

 recognized in the formation. 



6. Ottrelite (Masonite, Chloritoid). 



This species belongs to the same mineralogical group as the preceding, but 

 differs from it in many physical properties no less than in chemical composition. It 

 is in disseminated scales of a blackish-green color, whose breadth is rarely more than 

 a quarter of an inch. They present considerable resemblance to mica where seen on 

 weathered surfaces of the vein or on open joints of the rock, but are easily distin- 

 guished by their greater hardness and want of elasticity. It is chiefly confined to the 

 stony emery. In composition it differs considerably from the margarite, having above 

 20 per cent of protoxide of iron, together with 6 per cent of water, and stands in relation 

 to mica somewhat as emery does to corundum. 



'This nonalkaline feature of the contents of the vein, together also with the paucity of silica, 

 seem to be essential conditions of all the emery Teins. It is curious to remark how completely all 

 the other micas, as well as the feldspars, with the exception of the indianite, itself not abundant, are 

 excluded from the formation. 

 MON XXIX 9 



