462 



MINERALOGY. 



Oryetogno- 



y- 



e. Blunt-edged, as in gypsum and steatite. 

 f. Very blunt-edged, as in fullers earth and loam. 



III. THE ASPECT OF THE DISTINCT CONCRETIONS. 



The mpect Distinct Concretions are those portions into which 

 of the dis- cer t a in minerals are naturally divided, and which can 

 r'eUoni" ^ e se P arate( l from on e another without breaking 

 through the solid or fresh part of the mineral. They 

 are separated from one another by natural seams, and 

 frequently lie in different directions. When they are 

 very much grown together, the natural seams are 

 scarcely visible ; in such cases, however, they can be 

 distinguished by their different positions and resplen- 

 dent lustre. They have been confounded with crystals 

 and fragments, from both of which, as is evident from 

 the preceding definition, they are completely different. 

 Here we have to consider, 1. The shape of the distinct 

 concretions. 2. The surface of the distinct concretions ; 

 and, 3. The lustre nf the distinct concretions. 



VII. The Shape of ike Distinct Concretions. 



The shape Distinct concretions, in regard to shape, are clistin- 



of the dis. guished into granular, lamellar, and columnar. 



tinct con- ] . Granular distinct Concretions. 



retions. When the concretions are tessular, or have their 

 length, breadth, and thickness nearly alike, they are 

 said to be granular. It is the most frequent form of 

 the distinct concretion. They are distinguished accord, 

 ing to shape and magnitude. 



A. In regard to shape, they are 



a. Round granular, which is either 



. Spherical, as in pea-stone and roe-stone. 



P. Lenticular, as in red granular clay-iron-stone. 



y. Date-shaped, which is of a longish round shape, 



as in quartz-rock near Cullen in Banfi'shire, and 



at Prieborn in Silesia. 



b. Angttlo-granular, which is either, 



0. Common angulo- granular, as in galena or lead- 

 glance, and is very frequent. 



0. Longish angulo- granular, as in red hematite and 

 zeolite. 



B. In regard to magnitude, into 



a. Large granular, in which the size exceeds that of 

 a hazel-nut, as in galena or lead-glance, blende, 

 and zeolite. 



b. Coarse granular, in which the size varies from the 

 size of a hazel-nut to that of a pea, as in galena or 

 lead-glance, blend, mica, and pea-stone. 



c. Small granular, in which the size varies from that 

 of a pea to that of a millet-seed, as in galena or 

 lead-glance, pea-stone, roe-stone, and black blende. 



2. Lamellar distinct Concretions. 



In the lamellar distinct concretions, the length and 

 breadth are nearly alike, and more considerable than 

 the thickness. They occur frequently, but not so of- 

 ten as the granular concretions. 



They are distinguished in regard to direction and 

 thickness. 

 A. In regard to direction, they are 



a. Straight lamellar, which is either, 



cc. Quite straight, as in straight lamellar heavy. 



spar, or 

 /3. Forlification-nise bent, as in amethyst. 



b. Curved lamellar, which is either, 



a,. Indeterminate curved lamellar, when it is not 

 curved in any particular direction, as in specu- 

 lar iron-ore or iron-glance. 



ft. Reniform curved lamellar, as in red and brown Oryctogno. 

 hematite, and native arsenic. sy. 



y. Concenlrical curved lamellar, when they are ^ P- Y"~ > ' 

 disposed around a central point. It is divided 

 into spherical, as in calcedony and basalt, and 

 conical, as in calc-sinter and brown hematite. 

 B. In regard to thickness, into 



a. Very thick lamellar, when the concretions are up- 

 wards of half an inch thick, as in amethyst, and 

 galena or lead-glance. 



b. Thick lamellar, when the thickness varies from 

 half an inch to a quarter of an inch. 



c. Thin lamellar, when the thickness varies from a 

 quarter of an inch to a line, as in straight la- 

 mellar heavy-spar and calcedony. 



d. Very thin lamellar, from that of a line, or the 

 one-twelfth of a line, to the smallest thickness visi- 

 ble to the naked eye, as in straight lamellar heavy- 

 spar, native arsenic, and specular iron-ore or iron- 

 glance. 



8. Columnar or Prismatic distinct Concretions. 



In the columnar concretions, the breadth and 

 thickness are inconsiderable in comparison of the 

 length. 



They are distinguished in regard to direction, thick- 

 ness, shape, and position. 



A. In regard to direction, they are, 



a. Straight columnar, as in schorl and calcareous 

 spar. 



b. Curved columnar, as in columnar clay iron-stone. 



B. In regard to thickness, they are 



a. Very thick columnar, when the thickness exceeds 



half an inch, as in amethyst and prase. 

 6. Thick columnar, from half an inch to a quarter of 



an inch, as in quartz and calcareous spar. 



c. Thin columnar, from half an inch to the twelfth 

 of an inch, as in columnar clay iron-stone and 

 schorl. 



d. Very thin columnar, when it does not exceed the 

 twelfth of a line, as in schorl. When the concre- 

 tions become very minute, a transition is formed 

 into the fibrous fracture. 



C. In regard to shape, they are, 



a. Perfect columnar, when the length is considera- 

 ble, and the thickness uniform from one end to 

 the other, as in calcareous-spar and schorl. 



b. Imperfect columnar, when the concretions are in 

 general short, and sometimes thick in the middle, 

 sometimes at the extremities, as in amethyst and 

 specular iron-ore or iron-glance. It passes into 

 granular. 



c. Cuneiform columnar, when the concretions be- 

 come gradually narrower towards one extremity, 

 as in calcareous-spar and quartz. 



d. Ray-shaped columnar, when the columnar con- 

 cretions are compressed, as in specular iron-ore or 

 iron-glance. It passes into radiated. 



D. According to the position, they are 



a. Parallel, as in amethyst. 



b. Diverging, as in schorl. 



c. Promiscuous, as in calcareous-spar and arsenical, 

 pyrites. 



It may be remarked, that when the concretions oc- 

 cur very much on the great scale, as is the case in 

 rocks of the trap formation, a slight alteration of terms 

 is used. Thus, in place of granular we say massive, 

 tabular tor lamellar, and always use columnar, never 

 prismatic. 



