18 BULLETIN 691, U. S. DEPARTMENT OF AGRICULTURE. 
to a temperature of from 275° F. to 325° F. and the mineral 
aggregate to a temperature of from 250° F. to 300° F. before 
mixing. The mixture should contain from 7 per cent to 11 per 
cent bitumen and will vary within these limits with the character 
of the rock and the exact grading of the aggregate. The hot mix- 
ture should be deposited in a uniform layer over a concrete or 
well compacted broken stone or gravel foundation to such depth 
that it will yield a uniform layer 2 inches in thickness after 
thorough rolling. A seal coat should not be necessary on this 
type of pavement, which is usually maintained by patching with 
the same mixture used in construction. 
