696 CROSS, IDDINGS, PIRSSON, AND WASHINGTON 



of the grain of rocks, remembering that in finer-grained rocks the 

 millimeter is oftener the unit of measurement, especially in micro- 

 scopic work: 

 Meter- grained rocks, when the average size of the crystals is over 



I meter. 

 Decimeter- grained rocks, when the size of the crystals is from i to 



ID decimeters. 

 Centimeter- grained rocks, when the size is from i to 10 centimeters. 

 Millimeter- grained rocks, when the size is from i to 10 millimeters. 



Millimeter-grained rocks would include medium-grained and 

 slightly coarse-grained rocks. 

 Decimillimeter- grained rocks; those in which the average size of 



the crystals is from o. i to i . o millimeter. 

 Micron- (millimillimeter-) grained rocks; those in which it is from 



I to 10 microns, from o.ooi to o.oio millimeter. 

 Decimicron- (decimillimillimeter-) grained rocks; those in which it 



is from o.i to i.o microns, o.oooi to o.ooio millimeter. 

 The terms just described may be found useful in describing 

 rocks in a general manner, but each includes a wide range of varia- 

 tions in the size of grain, which in the extreme reaches a ratio of 

 10:1 for the diameters of the crystals, which is a ratio of 100:1 for 

 areas of crystal sections; differences which might be found in the 

 crystals of porphyritic rocks. It follows that the grain of the rocks 

 that may be described by any one of the terms mentioned — as, for 

 example, milUmeter-grsiiiied — is not always of the same order of 

 magnitude^ since in one case it may be eight or nine times larger in 

 diameter than in another. 



III. Fabric 



The arrangement of the crystalline parts of a rock, or of the crys- 

 talline and glassy parts when glass is present, which we have called 

 the fabric, depends on the relative sizes of the parts, on the shape of 

 the crystals, and on the positions with respect to one another and to 

 the glass base when present. The significance of these factors will 

 appear upon further consideration. 



Relative size of crystals. — While it never happens that all crystals 

 in a rock are of one size, it often happens that they are approximately 



