UNITED STATES DEPARTMENT OF AGRICULTURE 
I BULLETIN No. 348 
Contribution from the Office of Public Roads and Rural 
Engineering, LOGAN WALLER PAGE, Director 
sZfr^hj-u 
Washington, D. C. 
PROFESSIONAL PAPER 
April 4, 1916 
RELATION OF MINERAL COMPOSITION AND ROCK STRUCTURE 
TO THE PHYSICAL PROPERTIES OF ROAD MATERIALS. 
By E. C. E. Lord, Petrographer. 
CONTENTS. 
Page. 
Introduction 
Examination and classification of rocks for 
road making 
Mineral composition of rocks for road mak- 
ing 
Physical properties of road materials 
Page. 
General relation between the physical prop- 
erties of rocks and their mineral composi- 
tion and structure 11 
Effect of secondary minerals on the physical 
properties of rocks for road building 13 
Summary and conclusion 24 
INTRODUCTION. 
The importance of selecting proper materials for the construction 
of macadam roads has become generally recognized and has led to a 
careful study in this office of a large number of rocks in order to 
properly determine their mineral composition and road-making prop- 
erties. The earlier results of these investigations, which were pub- 
lished in 1911/ have demonstrated that certain physical properties 
are in a general way characteristic of definite rock types and may be 
attributed to the peculiar arrangement and composition of the rock- 
forming constituents. With a largely increased number of samples 
available for examination it has been found possible to correlate these 
relations more exactly, and in the present publication the results 
obtained will be presented with special reference to the effect of 
secondary minerals on the physical properties of rocks for road 
building. 
In considering these results it should be noted that owing to the 
complexity of the subject and of the possible effect of many variables, 
it has not been possible to develop definite laws. The discussion of 
the effect of secondary minerals has therefore been based necessarily 
i Examination and Classification of Rocks for Road Building, U. S. Office of Public Roads Bui. 37. 
17715°— Bull. 348—1 
