E. Stanley Jerons — Scale of Hock Texture. 255 



III. — A Numerical Scale of Textuke for Kocks. 

 By H. Stanley Jevons, B.A. 



WHEN reading through descriptions of hand-specimens of 

 igneous rocks I have often found a difficulty in forming 

 an exact mental picture of the appearance of the rocks because of 

 the very meagre information usually given as to the coarseness or 

 fineness of their texture. The following simple method would, 

 I believe, to a great extent obviate the difficulty now experienced in 

 giving such information. 



Since the degree of texture of a rock depends on the average 

 volume of its constituent crystals, the theoretically correct method 

 of expressing it is by stating the latter in some standard measure. 

 The determination of this average volume is, however, practically 

 impossible, so we fall back on the next best method for its 

 expression, that of measuring the average area of the individual 

 crystals exposed on any surface of the rock. One way of doing 

 this is by counting the number of crystals in a given area, but it 

 is a very cumbrous process even with the best appliances I could 

 devise. The method to be described consists in directly measuring 

 the length of a number of crystals and calculating their average 

 length. Its fundamental idea is the average area of the crystals, 

 but this is expressed by means of their average linear dimensions, so 

 that the figures, though originally lengths of crystals, are primarily 

 symbols for their areas, a fact which it is important to bear in mind 

 when a crystal of irregular outline is being measured. They are 

 thus not proportional to the areas they represent, but to their square 

 roots. As it is desirable to have a conventional usage on such 

 points, I have chosen the length as the dimension for measurement, 

 though theoretically any other would have done, and the millimetre 

 as the unit in which it is expressed, because of its convenient 

 size. As the same unit will probably be used by all, it will be 

 imnecessary to mention it each time, 3, for instance, being understood 

 to mean 3 mm. 



The only apparatus required is a small scale, preferably of ivory, 

 about eight centimetres long, divided into millimetres throughout 

 and into half-millimetres for two centimetres from both ends.^ To 

 measure the degree of texture with this, one first studies the surface 

 of the hand-specimen, fixing with the eye on several crystals which 

 appear to be of about the average size, and then one measures these 

 to the number of about a dozen, taking the average. If the crystals 

 are oblong the diagonal should be measured, if oval the greatest 

 diameter (when lath-shaped the fact must be mentioned) ; but if 

 they are very ragged one must simply measure the conspicuous 

 parts, for it is the size of these, after all, which gives the rock its 

 appearance of a coarse or fine texture. Some examples later on will 

 perhaps render these directions clearer. The size of the individual 

 crystals is fairly constant in some rocks, but in most the majority of 

 the crystals will be of about the same size, while the rest will vary 



1 Such a scale may be liad of Messrs. Stanley & Son, Great Turnstile, Loudon. 



