538 REPORT OF NATIONAL MUSEUM, 1890. 



ceous rocks often display a pronounced fissile structure which is due as 

 a rule to pressure, and in no way dependent upon the origiual bedding. 

 Such, when splitting with sufficient ease into thin smooth slabs, are 

 used for roofing and other purposes and known simply as slates. The 

 cause of this slaty cleavage will be explained more fully under the head 

 of dynamical geology. These cleavable rocks have been actually met- 

 amorphised by the pressure to which they have been subjected, and are 

 therefore mainly exhibited with others of the metamorphic group. A 

 few specimens are here placed to show the easy transitions from true 

 fragmental rocks to the crystalline schists. 



Kaolin is a very pure form of the hydrous silicate of alumina, formed 

 from the decomposition of feldspathic rocks. It is, in its purest state, 

 as has already been explained, a chemically formed rock, a residu d 

 product formed by the decay of feldspars and the removal of the solu- 

 ble portions, the alkalies and iron oxides, by percolating water. It is 

 used in the manufacture of porcelain ware. (Specimens 34441,37240, 

 2879, 36029, etc.) 



Kaolin, mixed with more or less siliceous matter, iron oxides, and 

 other impurities, forms the well known common clay used by potters, 

 pipe makers, for tile and for brick making. (Specimens 38162, 27394, 

 36783, 36042, 37278, 27407, etc.) The names clayey marl and shale are 

 applied to indefinite admixtures of clay with calcareous and siliceous 

 matters. Catlinite or " Indian pipe-stone" is an indurated clay rock 

 formerly used by the Dakota Indians for pipe material. (Specimen 

 38373.) The name porcellainite has been given to a compactporcelain- 

 like rock consisting of clay indurated by igneous agencies. (Specimens 

 36101 and 3809.) The name Wacke is sometimes used to designate an 

 earthy or compact dark colored clayey rock resulting from the decom- 

 position in situ of basaltic rocks. (See specimen No. 73102, with coating 

 of hyalite, from Bohemia.) 



3. The calcareous group. — Here are brought together a small series of 

 fragmental rocks composed mainly of calcareous material, but of which 

 the organic nature, if such it had, is not apparent. These rocks form 

 at times beautifully brecciated marble. (See building and ornamental 

 stone collection.) Their structure may be best comprehended by re- 

 membering that the original beds, whether crystalline or amorphous, 

 whether fossiliferous or originating as chemical precipitates, have by 

 geological agencies been crushed and shattered into a million fragments, 

 and then, by infiltration of lime and iron-bearing solutions been slowly 

 cemented once more into solid rock. The actual amount of movement 

 of the various particles has in many cases been but slight, as will be 

 noted by reference to the collections. Specimen 72868 from the Sphinx, 

 Montana, furnishes a good example of the coarser varieties of these 

 rocks. 



4. The volcanic group — Tuffs. — Here are brought together a great 

 variety of fragmental rocks, composed of the more or less finely com mi- 



