GEOGRAPHICAL DISTRIBUTION. 



17 



Whenever a clay of high binding power is heated to a red heat, its 

 binding power, together with the absorbent porous structure which is 

 characteristic of the colloid, is totally destroyed. This is equally true 

 of these artificial preparations and of high-binding rock dust. It is 

 impossible not to conclude that the same structure is present in all 

 three cases. In Plate I reproductions of micro-photographs are shown 

 which are highly instructive. No. 1 is an active clay, and the illustra- 

 tion clearly shows the coherence of the finer noncrystalline particles. 

 No. 2 shows the same clay heated sufficiently to destroy the colloid 

 structure. Owing to incipient fusion, there has been some clumping, 

 but the free spaces discernible all over the field between the grains 

 show that there can be no question as to the destruction of the adherent 

 quality of the particles. 



GEOGRAPHICAL DISTRIBUTION AND CEMENTING VALUE OF 

 ROAD MATERIALS. 



Those who are interested in a scientific study of road materials will 

 undoubtedly acknowledge the importance of understanding the funda- 

 mental causes of all the useful qualities these materials possess. It is 

 natural, however, that the average man should desire more particularly 

 such information as will enable him to select the best material for his 

 purpose. If the actual results of service have not already been deter- 

 mined there is no alternative but an appeal to the testing laboratory. 

 A study of Table III will show that cementing value is not confined 

 to aiw particular species of rock. The highest trap tested among a 

 large number of samples had a cementing value of 500, the lowest a 

 value of 1. In the same way the limestones will be found varying 

 from 231 to 4=. 



TABLE III. Range .of cementing value in different rock species. 



518604 2 



