1916] KUEAL ENGINEEEING. 85 



The results obtained in these investigations are taken to indicate the fol- 

 lowing : 



" Igneous and nonfoliated metamorphic rocks, owing to a preponderance of 

 hard silicate minerals combined with gi*eater uniformity in structure, are more 

 durable than other road-making materials, finer-grained varieties offering 

 greater resistance to abrasion than coarse-grained types. The resistance to 

 wear of igneous and metamorphic rocks, containing an abundance of quartz, 

 hornblende, augite, epidote, and garnet, is greater than that of similar rocks 

 rich in mica, chlorite, serpentine, and calcite. 



" Foliated metamorphic rocks, owing to the parallel arrangement of their 

 mineral constituents, are, as a rule, deficient in toughness, and therefore not 

 well adapted to road construction. Sedimentary rocks are usually deficient 

 in wearing properties, except in the case of highly indurated sandstones, con- 

 taining a moderate amount of siliceous clay, cement, and limestones or dolo- 

 mites rich in quartz and having very little clay. 



"Rocks for road making break down under impact into fragments, the 

 shape and physical character of which are conditioned by mineral composition 

 and structure. 



" The effect of weathering is generally to lower the resistance to wear of 

 road materials, owing to the development of soft, in part colloidal, products of 

 alteration. Where the secondary minerals are harder and more crystalline 

 the wearing properties of the rocks are proportionately increased. 



" The cementing value of road materials is conditioned chiefly by the colloidal 

 products of rock decay and increases in a gAieral way proportionately with 

 these products, reaching a maximum in rocks free from quartz. The slaking 

 property of rock powders is dependent In the case of siliceous igneous and 

 metamorphic rocks chiefly on the physical character of the primary mineral 

 components, whereas in basic igneous rocks and sandstones it is caused to 

 a large degree by colloidal products of rock decomposition." 



See also a previous note by the author (E. S. R., 25, p. 890). 



A new penetration needle for use in testing bituminous materials, C. S. 

 Ree\^ and F. P. Peitchakd {U. S. Dept. Agr., Jour. Agr. Research, 5 {1916), 

 No. 24, pp. 1121-1126, pi. i ) .—Experiments conducted in the Office of Public 

 Roads and Rural Engineering are reported, as a result of which the following 

 conclusions are offered : 



" The No. 2 sewing needle which has heretofore been used for the penetration 

 test can not be taken indiscriminately, but must be carefully selected and stand- 

 ardized. There is no recognized established standard with which new needles can 

 be compared, and it is not feasible to accurately describe the dimensions of a 

 parabola needle. The so-called standard needles furnished with penetration 

 machines may vary among themselves. 



" The writers have designed a needle which gives results in close accord with 

 existing standards and has, moreover, the advantage of being accurately de- 

 scribed and easily reproduced. The neetlle is made by placing a 2-in. length of 

 0.041-in. annealed-steel drill rod in the chuck of a high-speed lathe and By 

 means of a fine sharp file turning the end to a sharp point having a i-in. taper. 

 When it has been made as smooth and sharp as possible by this means the needle 

 is tempered, then ground to a sharp point with a good stone, after which it is 

 smoothed and polished with emery dust, crocus cloth, and rouge, and finally held 

 carefully on a buffing wheel. The finished needle should be sufficiently smooth 

 and sharp to enter and pass through a piece of ordinary writing paper without 

 sticking or friction. This new needle must have as sharp a point and smooth a 

 surface as any sewing needle. The important thing is to have the taper straight, 



