70 A NATURALIST IN THE GREAT LAKES REGION 



tabulation will give the distinguishing characters of the com- 

 monly occurring minerals. A few terms need definition at 

 the outset as they will be used in the descriptions. Certain 

 minerals like calcite, galenite, and feldspar have a tendency to 

 split easily along certain planes so that the pieces are bounded 

 by smooth surfaces. This is known as cleavage (Fig. 47). 

 Galenite cleaves into cubes, feldspar into blocks of which the 

 opposite faces are parallel while the adjacent faces meet in 

 angles that are either a little more or a little less than right 

 angles. Fracture is the surface produced when a mineral breaks 

 in any other direction than along its cleavage planes. Thus flint 

 has a conchoidal or shell-shaped fracture. Streak is the color 

 given when the mineral is rubbed on a piece of unglazed porcelain 

 or when it is scratched. Luster is the appearance when light is 

 reflected from the surface of the mineral. This may be vitreous 

 like the reflection from a freshly broken piece of glass, resinous, 

 pearly, silky, etc. terms that carry their own meaning plainly. 

 One of the chief means of distinguishing minerals is the hardness. 

 So important is this that a definite scale of hardness has been 

 agreed upon. Thus, talc has a hardness of i, gypsum 2, calcite 

 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 

 9, diamond 10. For ordinary purposes the hardness may be 

 indicated in a less exact way, still the numbers given after each 

 mineral indicated the hardness on this scale. 



Sedimentary rocks. As we have seen in the previous chapters, 

 sedimentary rocks are originally laid down as beds (i) of cal- 

 careous materials like shells, corals, or their water-worn frag- 

 ments, (2) beds of angular rock fragments, (3) of gravel, (4) of 

 sand, (5) of clay, or (6) of plant debris. Later these, by means 

 of cement, by pressure or by heat, acting singly or in unison, are 

 transformed to stone. The calcareous material, really calcite or 

 calcium carbonate, makes limestone a rock that can be scratched 

 with a knife, effervesces with an acid, and often contains fossils. 

 Angular rock fragments cemented together form a breccia, while 



