220 OUTLINES OF FIELD-GEOH ' PA> 



tllinc grains which lie close together, when a good 

 deal more force will be required to scratch the cleavage 

 face of one of these grains. The test of relative hardness 

 serves usefully to distinguish various compact rocks from 

 each other, when there is no obvious internal difference 

 by which the eye could discriminate between them. 

 Some pale dull felsites resemble certain limestones 

 and dolomites, but their much greater hardness in a 

 fresh state serves at once to mark them. Again, some 

 exceedingly compact dark volcanic rocks, such as basalts 

 and aphanites, might at first be mistaken for clay-band 

 ironstones or black limestones or mudstones, but they 

 will easily be distinguished by their superior hardiuss 

 and weight 



2. Sptciju Gravity. Determinations of the specific 

 gravity of rocks may be usefully made when the geologist 

 has brought home some rock-specimens about the com- 

 position of which he may be in doubt. By this means, 

 for instance, he can readily separate his limestones which 

 have a specific gravity of about 2.72 from his dolomites 

 which are heavier (2.85). In like manner, he can dis- 

 criminate acid from basic igneous rocks, and may form a 

 shrewd guess as to what special rock-species his specimens 

 should be assigned. The most convenient instrument 

 for determining the specific gravity of rocks in the pro- 

 gress of field-work is undoubtedly Walker's balance, 1 

 which consists of a small steel-yard divided into inches 

 and tenths. The specimen to be determined is hung by 



1 This useful and portable instrument is made by G. Lowdon, 

 Reform Street, Dundee, and can be obtained through any instrument- 

 maker. 



