Granite and syenite are the principal rocks of which setts are made for paving carriageways, or streets in large towns, where vehicular traffic is heavy and continuous. The trap rocks, greenstone or whinstone, and basalts are also greatly used for this purpose, but their durability, unless in isolated instances, cannot be compared with that of certain granites and syenites.

117. The Mineralogical Composition of different Rock Materials for Roads.-The materials employed for road metal are prepared from different kinds of rock, the principal of which are Granites, Syenites, Diorites, Basalts, Dolerites, Diabases, Quartzites, Mica-schists, Limestones, Ragstones, Sandstones, Flints, and Gravel taken from pits, river beds, or the sea-beach.

The mineralogical composition of rocks varies considerably, while a great difference exists in the chemical stability of many of the minerals composing them. The mineralogical peculiarities or qualities combined in a typical road stone depend principally on the composition, and to a great extent on the manner in which the different minerals are aggregated together, such as the crystalline texture and the nature of the base or matrix, forming a kind of paste or cement in which the crystals are imbedded.

The conditions under which rock masses have been hardened can be tolerably well ascertained by noting the degree of crystallization which they have undergone. This depends to a considerable extent upon the slowness or rapidity with which they have cooled, and the depth at which consolidation has taken place. Generally speaking, each mineral has its own temperature point at which it crystallizes. When rock has chilled rapidly it appears glassy, the crystals not having had time to form; while, on the other hand, if the cooling process of the molten mass has been extremely slow, the chemical elements have had time to assort and arrange themselves into chemical compounds, and in obeying the laws of crystallization to assume special forms.

118. Granite.*-Granite is composed essentially of felspar, mica, and quartz aggregated together in a thoroughly crystalline-granular manner. The felspar is either orthoclase (potash felspar), albite (soda felspar), or oligoclase (soda-lime felspar), the former generally prevailing, the latter being found in smaller quantities.

The predominant mica in granites is the muscovite (potash) variety; the dark brown or black variety biotite (magnesian mica) is also found, while lepidomelane or haughtonite is of rare occurrence. The quartz or pure silica sometimes forms a paste in which the other minerals are embedded, and occasionally the component ingredients are found as independent crystals. The other accessory minerals, beryl, topaz, and tourmaline, are found in cavities of the granite when the minerals occur as independent well-formed crystals.

* The description of some of the rocks mentioned in the immediately following paragraphs is partly taken from the article on 'Geology' in the Encyclopædia Britannica by Professor Geikie.

The texture of granites varies considerably, from coarse crystalline masses to fine granular rocks, which pass into elvanite or felsite, and form an intermediate stage between granite and quartz-porphyry.

Of the different kinds of granite, those which are compact and finegrained, and composed of orthoclase and muscovite, may be taken as a reliable material for road metal. The quartz and mica are practically unaltered chemically; the felspar, however, especially when oligoclase is present, decomposes rapidly into clayey mud on being subjected to the disintegrating influence of air and water. Granites vary in colour from a grey hue when the crystals of felspar are dusky white, to red when they are coloured by the presence of iron.

Quartz-Porphyry.-Certain rocks, closely allied to, and sometimes called granites, are Quartz-porphyry, Felsite-porphyry, Felstone, and Felsite. These are composed of a mixture of orthoclase and quartz as the essential constituents, little or no mica being found in their composition. When the quartz forms conspicuous crystals the rock is called quartz-porphyry, but when the felspar and quartz are intimately mixed so as to present a homogeneous matrix the rock is termed felsite-porphyry, felstone, or felsite; they are generally very compact in texture or even flinty in appearance. Triclinic felspars occasionally occur in these rocks, while the minerals mica and hornblende accompany the essential constituents; apatite, magnetite, and pyrite being at times met with as accessories. The colours of these rocks vary from flesh-red, purple, yellow, to slate-grey, depending chiefly on the felspar, while dark-grey, brown, and greenish tints are imparted by the presence of mica or hornblende.

119. Syenite. This name is now restricted to rock consisting essentially of a mixture of orthoclase and hornblende, but which was formerly applied to granite when the mica was replaced by that of hornblende. In addition. to the component elements mentioned, the minerals plagioclase, quartz, and mica are occasionally added as accessories. The durability of syenite is greater when quartz and hornblende predominate; felspar and mica are weak and of a perishable nature if present to any great extent, and disintegrate rapidly, especially when the crystals are large and have a dull appearance. The colour of syenite varies from pink to grey. In most cases the rock forms an excellent stone for road-metalling purposes.

120. Syenite-porphyry contains little or no quartz, but has a little plagioclase, and sometimes hornblende and biotite or black mica; it is an orthoclase rock, and is usually termed orthoclase-porphyry. Syenite-porphyry is closely allied to syenite, being associated with that rock much in the same way that elvanite is with granite.

121. Diorite.-Diorite is the term applied to greenstones or whinstones, and consists of a crystalline mixture of oligoclase or some allied felspar and hornblende, with magnetic iron and apatite. These rocks occur in an

eruptive form, and under conditions similar to syenite; the more dark compact varieties are sometimes called aphanite, while those containing free quartz are known by the name of quartz-diorite. These rocks form a large proportion of the material used for road repairs in Scotland, and when the component minerals are fine-grained and compact they make very durable and satisfactory road metal. On the other hand, those which contain soda and lime felspar, combined with a coarse texture, quickly decompose and form clay, which creates on the roads during wet weather large quantities of mud. Hornblende andesite is a rock similar to diorite, the felspar being plagioclase; when it contains free quartz it is called dacite.

122. Basalt. This rock consists of a triclinic felspar, generally labradorite, together with augite, olivine, and magnetic or titaniferous iron, with apatite and sometimes sanidine or nepheline. The augite in basalt plays a similar part to that taken by hornblende in the rocks just described; it is also a mineral constituent of most of the following series. Basalt is a volcanic rock of great importance, and occurs as dykes, veins, intrusive bosses and sheets, while sometimes it presents a columnar structure. It is close-grained and compact, the dark colour being chiefly due to the large proportion of magnetite, which also contributes to its high specific gravity. The term dolerite is given to the more coarsely crystalline varieties of basalt, the durability of which is very variable.

123. Diabase is a name given to rocks consisting essentially of the same mineral constituents as basalt; they are, however, much older, and having been longer exposed to metamorphic processes are consequently less fresh than basalts. Most varieties of basalt are eminently suitable as material for macadam; they are generally durable and hard, combined with the other qualities upon which depends the value of road metalling.

All the rocks described are of igneous origin, their internal characteristics being either purely crystalline, consisting entirely of crystals interlaced with each other as in granite, or half-crystalline composed of a noncrystalline amorphous matrix with crystals scattered through it; the latter division embraces most of the eruptive masses.

124. Among metamorphic rocks the quartzites and greywackes are best; many of these have a sufficient admixture of felspar, which tempers the natural brittleness of the quartz

125. Mica Schist is found in mountainous regions, and is much used as a road material in the Highlands of Scotland and in some parts of Wales. It is composed essentially of quartz and mica, disposed in layers which give it a stratified appearance. On this account it is not a suitable stone for repairing roads which have heavy traffic passing over them. This description of stone binds well, however, and makes a very good surface on roads with light traffic, but creates much mud in wet weather, while at the same time it has the advantage of drying quickly after a fall of rain.

126. The Carboniferous or Mountain limestones are greatly used in some counties in England, Wales, and Ireland for macadamizing purposes; they are very suitable for roads with light traffic, and make a good surface, but create large quantities of mud in wet and dust in dry weather. It can only be applied economically on the class of roads mentioned, and where the climate is equable; it is, however, rendered practically useless as a metalling material by alternate frost and thaw.

Ragstone or Kentish-rag is a limestone occurring with alternate layers of sandstone known as hassock, and belongs to the Lower Greensand formation. It is greatly used in Kent as a road material and for kerbs and channels.

127. Sandstones are greatly used, especially the harder kinds, for bottoming roads, but as a rule are too soft for metalling purposes; they also absorb a considerable amount of water. Many sandstones are chemically indestructible, but being loosely aggregated together are, under the wear of wheel traffic, quickly pulverized, the dust having little or no binding qualities. The millstone-grits are the best of this class, and may be used for surface repairs on roads with moderate traffic; they are, however, liable to rapid disintegration during drought and frost.

128. Flints are extensively used in the chalk districts of England; they give excellent results so far as a good surface is concerned, but are too brittle for heavy traffic. Those gathered from the surface of the land are superior to those dug from gravel-pits, or, as they are termed, quarry flints. The binding properties of flints are small, for which reason an admixture of ragstone or hard chalk is sometimes employed to produce a better road surface.

129. Gravel and stones gathered from the surface of the land are extensively used in districts where more suitable materials are difficult to obtain. The gravel, whether taken from pits, river beds, or the sea-beach, is all more or less rounded and worn. Stones of this kind, when used as metalling, make a very rough surface, and being of unequal hardness they wear very irregularly. Land-stones are not, for similar reasons, a suitable material for repairs, and their use should only be tolerated when a more suitable kind of stone is not available. Ironstone and copper slag are at times employed, especially for bottoming roads; as a metalling material they possess no binding properties.

Among stones of the same kind that which bas the greatest specific gravity is invariably the strongest, but great difference may exist in the durability of stones of the same kind and presenting little difference in appearance.

Table XI. gives the chemical analyses of the more important rocks described in the preceding paragraphs.

130. The Requisites in a Road Stone.-In selecting a material for