60 CRYSTALLOGRAPHT. 



3. Consisting of scales or lamella?. 



a. Plumose, Laving a divergent arrangement of scales, as 

 seen on a surface of fracture; e. g., plumose mica. 



b. Lamellar, tabular, consisting of flat lamellar crystalline in- 

 di\iduals, superimposed and adhering. 



c. Micaceous, having a thin fissile character, due to the aggi* 

 gallon of scales of a mineral which, like mica, has emineai 

 cleavage. 



d. Septate, consisting of openly-spaced intersecting tabular 

 individuals ; also divided into polygonal portions by reticulat- 

 ing veins or plates. A septarium is a concretion, usually flat- 

 tened spheroidal in shape, the solid interior of which is inter- 

 sected by partitions ; these partitions are the fillings of cracks 

 in the interior that were due to contraction on drying. When 

 the surface of such septate concretions has been worn off, they 

 often have the appearance of a turtle's back, and are sometimes 

 taken for petrified turtles. 



4. Consisting of grains. Granular structure. A massive 

 mineral may be coarsely granular or finely granular, as in 

 varieties of marble, granular quartz, etc. It is termed saccha- 

 roidal when evenly granular, like loaf sugar. It may also be 

 cryptocrystalline, that is, having no distinct grains that can 

 be detected by the unaided eye, as in flint. The term crypto- 

 crystalline is from the Greek for concealed crystalline. Aphani- 

 tic, from the Greek for invisible, has the same signification. The 

 term ceroid is applied when this texture is connected with a 

 waxy lustre, as in some common opal. 



Under this section occur also globidar, botryoidal, and mam- 

 miliary forms, as a result of concretionary action in which no 

 distinct columnar interior structure is produced. They are 

 called £)isolitic when in masses consisting of grains as large as 

 peas (from the Latin pisum, a pea), and oolitic when the grains 

 are not larger than the roe of a fish, from the Greek for egg. 



5. Forms depending on mode of deposition. — Besides the 

 above, there are the following varieties which have ccme from 

 mode of deposition : 



a. Stalactitic, having the form of a cylinder, or cone, hang- 

 ing from the roofs of cavities or caves. The term stalactite is 

 usually restricted to the cylinders of carbonate of calci im hanging 

 from the roofs of caverns ; but other minerals are said to have 

 a stalactitic form when resembling these in their general shape 

 and origin. Chalcedony and brown iron ore are often stalacti- 

 tic. Interiorly the structure may be either granular, radiately 

 fibrous, or concentric. 



