GEOLOGICAL DISTRIBUTION OF PAVING BRICK MATERIAL IN ILLINOIS.
What is a Paving discussing the distribution of pav ing brick materials it seems best to at least attempt to define what a paving brick is.
Pages enough to make a fair-sized library have been written in the effort to describe the properties which a paving brick should possess and to formulate a series of tests which would enable a man always to select the best among the samples submitted to him, but no satisfactory con clusion has yet been reached. The chapter by Professor Talbot which appears in this volume is an admirable presentation of our knowledge on this subject; but all that is really known may, I think, be condensed into the following definition : A paving brick is a rectangular block of burned clay which possesses in a prominent degree the properties of hardness and toughness. It must be hard in order that it may wear as slowly as possible under the severe abrasion to which it is subjected. It must be tough so that it will not be broken or crushed by the shocks which it receives.
While every paving brick must possess both the properties named some lack in toughness is not so serious a matter as a lack in hardness, if the pavement is properly laid, because in that case all surfaces but one are thoroughly supported and it would require an unusually severe shock to shatter it. In the streets of Champaign and Urbana are several miles of pavements made from brick so brittle that they required the most careful handling to place them in the pavement unbroken, yet they have withstood wear, unusually severe for a country town, for more than a decade without serious injury. These brick were made from ordinary surface glacial clay and burned hard. They were so brittle that most of them would have made a very poor showing in a rattler. If, then, the brick are very hard and are properly supported on the bottom, sides, and ends, we can overlook some deficiency in the way of toughness, although of course every effort possible should be made to secure as great a degree of toughness as possible.
What is a Paving Brick order to answer this question in telligently we must know what properties a clay must possess and what changes it must undergo in order to gain the desired degree of hardness and toughness. We do not yet know enough about the pyro-chemical
behavior of clay to enable us to answer either of these questions with confidence.
We know, or think we know, that vitrification tends to harden the brick and that proper regulation of the rate of cooling tends to make it tough. We also think we know that a mixture of ingredients which fuse at different temperatures is better than a mass of uniform com position, and that if the mixture while possessing the necessary degree of plasticity has its grains so firmly cemented that they will go through the processes of preparation and reach the molding machines as a mix ture of particles varying in size from coarse to very fine it will enable us to reach the desired result more cheaply than would otherwise be possible.
In consequence of this we naturally look for an impure shale as the most available material for the manufacture of pavers, first, because the impurities it contains will probably give us the desired range in fusi bility, and second, because shale after all is nothing but clay whose particles have been cemented and which by careful crushing can be converted into a mass which while having the necessary plasticity, will at the same time contain grains varying widely in size and density. In consequence of this many people have come to believe that shales furnish the only available material from which pavers can be made, and that shales from the coal measures are much more likely to make good pavers than those from any other horizon.
This opinion seems not to be well founded, for there is no good reason why coal measure shales should differ in any way from those of any other age. It seems to be true that the period of the coal measures was one of unusual instability, and that the frequent changes in condi tions brought corresponding changes in the character of the sediments which were deposited at a given point, so that .the content of sand in the coal measure shales varies more widely than is usual, and this in connection with the frequent occurrence of beds of coal at the base of the shale may account for the popular prejudice.