TEXTURE OF IGNEOUS ROCKS 



697 



of the same size, at least as far as the great majority of component 

 crystals are concerned. Crystals that are approximately of the same 

 size may be of the same order of magnitude. It becomes necessary, 

 then, to give this term a quantitative expression. Since identity 

 of size is not assumed, the question rises : What latitude of variation 

 in size is permissible in the petrographical use of the term — "The 

 same order of magnitude" or "hke magnitude," when apphed to 

 the crystals of a rock, or to sections of crystals in a rock section ? 

 If the crystals are nearly equidimensional, as in the accompanying 

 diagram, it would appear reasonable to consider those as having 

 like magnitude whose diameters varied within the ratio of 3 : 2 (Fig. i). 



<J / / V 



Fig. I Fig. 2 Fig. 3 



Those whose diameters vary as 2:1 or 3:1 are clearly of different 

 magnitudes (Figs. 2 and 3). The areas of the sections of the hmiting 

 sized crystals in the three cases are as 9:4, 4:1, 9:1, respectively, 

 and the actual volumes as 27:8, 8:1, 27:1. That is, the possible 

 limits are as follows: 



When the cross-sections of the crystals are not equidimensional, 

 but have different shapes, it is evident that the basis of comparison 

 is naturally the area of the cross- section : consequently such crystals 

 may be said to have like magnitude when the areas of their cross- 

 sections in a rock section or on the surface of the rock vary within 

 the limits of 3^:2^; that is, 9:4, or 2.25: i. It is interesting to 

 note in this connection that the range in orders of magnitude used 

 by astronomers in the comparison of stars is ^ I 100 or 2 . 51. 



