.o 



; 



40 THE TESTING OF ROAD MATERIALS. 



In describing the character of the material, the origin of the roc 

 as far as revealed by the microscope, has been observed. Thus rocks 

 with well-defined porphyritic or flow structure, such as rhyolite, ancle 

 site, volcanic tuffs, etc., are considered as effusive or volcanic, whil 

 deep-seated varieties such as granite, syenite, quartz-porphyry, etc. 

 are described as plutonic or intrusive as the case may be. When the 

 mode of origin is obscure, the general term "igneous" is used. Sand- 

 stones, limestones, etc., on the one hand, and quartzites, schis 

 gneisses, etc., on the other are treated as sedimentary and metamorphi 

 rocks respectively, while disintegrated material such as sands, clays 

 etc., are classed as sedimentary deposits. 



In describing the igneous rocks a very simple classification accord 

 ing to well-recognized types has been best adapted to the requiremen 

 of the laboratory. No attempt is made to subdivide these main fami- 

 lies except in rare instances where minerals foreign to the norm 

 composition of the rock occur in appreciable quantities. Thus 

 granite rich in hornblende would be termed hornblende-granite, or 

 syenite containing much augite, augite-syenite, etc. 



Finally the reports under general remarks call attention to th 

 origin of the secondary constituents as well as to the color, texture 

 structure, and other physical properties of the rock specimen likely 

 to be of use to road builders. The tabulated results on all samples 

 examined will be found in Table B of the Appendix. Microphoto- 

 graphs have been made from sections of some of the principal types 

 of rocks examined and some of these photographs, selected to show 

 the characteristic structure of various types, are reproduced in PI 

 I to V, inclusive. 



CHEMICAL ANALYSIS. 



now 







The purpose of the chemical analysis of road materials is to supple- 

 ment the petrographic identification and classification of rocks as well 

 a- to supply data which when taken in connection with the physical 



te-N may t'urni>h information as to the value of a given material. The 

 study and chemical examination of clays. marU. etc., has In-come an 

 important part of the laboratory work, not only because the manu- 

 facture of ceiiient>. drain tiles, and pipes is indirectly a road-building 

 problem, but al>o on account of recent efforts to const met practical 

 roads by mixtures of burnt and raw clay in sections of the country 

 where no stone is available. The rapid growth of intelligent intere-t. 

 which ix taking place all over the country, in the problems of road 

 building has brought about a greatly increased demand for laboratory 

 methods of investigation. The number of samples presented for 

 examination and the subsidiary value of the chemical examination in 

 most case> have led to modifications of the ordinary methods of rock 

 analysis which, in the experience of this laboratory, furnish a suffi- 

 cient degree of accuracy with the minimum expenditure of time. 



