EARTH; SAND-CLAY, AND GRAVEL ROADS. 39 



a good quality of topsoil. These names have considerable significance 

 in some localities, but may mean practically nothing in others. 



CLAY AND SAND. 



Nearly all clays, as they occur in nature, contain more or less sand. 

 Those in which the percentage of sand is sufficiently high to give 

 the material a character decidedly intermediate between sand and 

 clay are called natural sand-clay. Ordinarily, at least 40 per cent of 

 sand is required to produce this intermediate character in a pro- 

 nounced degree, and from 60 to 70 per cent usually is required to 

 produce a first-class sand-clay surfacing material, though these per- 

 centages depend to a considerable extent on the respective qualities 

 of the two constituent materials. 



In general, the quality of clay is much more uncertain than that 

 of sand and usually it is very difficult to determine in advance just 

 what results may be expected from the use of a given clay in a 

 sand-clay surface, except b}^ means of a service test. But some time 

 is required to make the service test, and, where none of the clay pro- 

 posed for use occurs in an existing road surface, it may be very de- 

 sirable to test the available material according to some quick method, 

 so as to exclude from the service tests all clays which would not be 

 likely to prove satisfactory. Some of the simplest methods for de- 

 termining the probable wearing qualities of clay, or of a natural 

 sand-clay mixture, by means of field inspection and laboratory tests 

 are described below. 



(1) To determine the proportion of sand present in clay, collect a 

 representative sample of the clay and dry it thoroughly. After 

 weighing the dry sample, place it in a vessel several times larger 

 than the sample and wash out the clay content by filling the vessel 

 repeatedly with water, agitating the clay and pouring off the muddy 

 water. When all the clay has been washed out, dry the sand and 

 weigh it. Then the proportion may be computed from the original 

 weight and the weight of the sand. If the material tested happens 

 to be of good quality of sand-clay, the weight of the sand will be 

 about two-thirds that of the original sample. 



(2) To compare the slaking qualities of two or more samples of 

 clays, make up each sample into several balls of the same weight, dry 

 out the balls thoroughly, and place them in water so that they will be 

 covered entirely. The balls which hold their shape longest after 

 being placed in the water have the highest resistance to slaking, and 

 the clay which they represent usually is to be preferred for use in the 

 road surface. If this test is to be at all conclusive, however, the 

 different samples should contain about the same percentage of sand, 

 and if there is much doubt about the sand content, it should be 



