other less important minerals. Microscopic accessories occur, but in such small quantities that they will not be taken up. In some varieties hornblende and pyroxene are considered essential.

Granite, according to Dana, consists of quartz, feldspar, and mica. Under this definition, no stone could be a granite unless it contained mica, but as the term is used commercially it includes syenite and gneiss and often porphyry. The order of the consolidation of rocks is an important factor in their structure. As a rule, in granite the minor accessory minerals crystallized first, taking their natural form. According to some authorities the ferromagnesian minerals came next, followed by the feldspars, and lastly by the quartz flowing in, filling all the interstices, making a complete and solid rock. Occasionally, however, quartz and feldspar are found completely intermingled, indicating that they crystallized at the same time.

While the character of a granite is determined principally by its essentials, the accessories have much to do with its quality. The color is generally fixed by the feldspar, but the mica is often a governing characteristic, the presence of muscovite making a granite light, while biotite has always the opposite effect. A large amount of quartz will make a granite hard and brittle, while too. much feldspar renders it softer and tougher, but more liable to decomposition. The susceptibility to polish and its ability to resist the action of the elements depend greatly upon the accessory components. Hornblende is a mineral which permits a granite to take a high polish, while pyroxene, being very brittle, often breaks out when a stone is being hammer-dressed, giving a pitted appearance to an otherwise smooth surface. Iron is detrimental, as by the action of the weather iron-rust is formed, and rains washing it over the surface of the stone produce stains upon any structure built of stone containing iron. The size of the particles of the minerals is important. The smaller the grains and the more evenly they are distributed, the better the stone will cut and be polished. The finer the grain the better satisfaction the granite will give in cut work. A fine-grained stone is compact in texture, excluding air and moisture, two agents that are constantly at work to destroy all minerals. Granite is divided into varieties according to the presence of its varying accessories.

Muscovite granite is so called from the mica being of the muscovite variety. It is not found in large quantities in this country, but is produced to some extent from the quarries of Barre, Vt.

Biotite granite is similar to the above except that the muscovite is replaced by biotite. On this account, while the former is always light in color, the latter varies from light to dark according to the quantity of mica or the color of the feldspar. This class of stone is often red, owing to the red feldspar. As a rule the stone is hard and tough. Good samples of it are found at Westerly, R. I., and Dix Island, Me.

Muscovite biotite granite stands between the two last described, having both varieties of the mica, and differing from them only in that respect. It is found at Concord and several other places in New Hampshire.

Hornblende granite is a variety in which the characterizing accessory is almost entirely hornblende. Biotite is, however, generally found upon a microscopic examination. When the mica cannot be discovered by the unaided eye the name hornblende" is given to the variety. Examples of this are found at Peabody, Mass., and Mt. Desert, Me.

Hornblende-biotite granite is distinguished from the above in that it contains as essentials quartz and feldspar with both hornblende and biotite. This combination gives a dark and sometimes an almost black granite, capable of receiving a fine polish.

Examples of this stone are found at St. George, Me., Cape Ann, Mass., and at Sauk Rapids, Minn.

One important property that is possessed by all granites is that of splitting more easily in one direction than another, so that it is easy to get out blocks large or small with practically parallel sides. This property is generally called rift or cleavage. It was caused by pressure before the rock was consolidated. The rift is always perpendicular to the line of pressure. When a stone is resting upon a face parallel to its cleavage plane it is said to be lying on its bed, and the face at right angles to the bed is called the edge. Rift is governed by the amount of pressure and the grain of the stone, so that while all granites have a rift they do not have. it in the same degree. The finer-grained granites have the best rift, decreasing as the grains increase, so that a coarse-grained

variety is apt to be bunchy and requires considerable dressing to bring the faces of the block to a plane surface. This fact is well known to quarrymen, and an experienced hand will easily and quickly tell the character of the rift by the general appearance of

the stone.

Although it has been said that granite breaks more easily in one direction than another, on account of its peculiar structure it can be broken into blocks of almost any shape by skilled workmen with a stone-hammer, or with proper wedges and lewises if a large and irregular block be required. By this method the dividing force is exerted in whatever direction desired by inserting the wedges and lewises into holes drilled for the purpose, when by lightly driving the wedges in succession the quartz which is holding the other crystals together is easily fractured and the granite breaks as desired. On account of this fact it is particularly adapted for pavingblocks and curbing, as it is cheaply and rapidly formed into the proper size and shape. Often a stone is barred from use as a paving material for the reason that so much work is required to get it down to specification size.

Gneiss is a variety of granite which differs from that just described only from the fact that its rift is caused by the greater portion of its mica being gathered in parallel planes so that the stone is easily broken along these planes. This is purely a physical difference, as chemically and mineralogically it is the same as granite proper. This arrangement of the mica weakens the stone appreciably when set on edge, a fact which is not true of the granites.

Dana defines gneiss as consisting of quartz, feldspar, and mica, and possessing cleavage planes.

Syenite, according to Dana, consists of feldspar and hornblende. with or without quartz. It will be noticed that the mica of granite and gneiss has disappeared and hornblende has taken its place. This latter mineral is hard and compact, varying considerably in its composition, but made up principally of silicate of magnesium and calcium, with some alumina and iron. It has its cleavage in two planes and is easily brought to a fine polish.

In 1787 Werner adopted the definition quoted above from Dana, but later German geologists have used the term syenite to designate rocks without quartz, differing only from granite in that

respect and consisting mainly of orthoclase feldspar in company with one or more minerals of the amphibole (hornblende) or pyroxene group. This combination has seldom been used or found in this country.

Porphyry. The mineral and chemical composition of the quartz porphyries is essentially the same as that of the granites, from which they differ mainly in their "porphyritic" structure. That is, the quartz has cooled first, thereby gaining a crystalline form so that the rock presents to the eye a dense compact mass of stone in which can be seen crystals of quartz alone or quartz and feldspar together. This structure characterizes all the rocks of this type. The ferromagnesian minerals are often confined to the elements of the earlier period of crystallization, while the original quartz is found in the acid types only, and is generally restricted to the ground-mass.

This change of structure prevents the formation of the rift so characteristic of the true granites. In composition it is generally about two-thirds silica.

Diabase (Trap-rock).-The essential constituents are plagioclase, feldspar, and augite, with nearly always magnetite and apatite in small proportions. The accessories are hornblende, biotite, olivine, etc. It is holocrystalline in form, but not often having perfect crystal outlines, as they are more or less distorted on account of interference during the process of formation. The feldspar generally crystallizes before the ferromagnesian constituent, the former being often found wrapped around by the augite. As a rule it is finer-grained than the granites. It varies in color according to its constituents from a dark gray to almost black. The rock is hard, compact, and tough, but not easily broken into regular shapes. It occurs in dikes, where the material in a melted state poured into the fissures already created and, cooling, there divided masses of the same character into separate and distinct parts. This is often seen in limestone formations in Maine. The best illustration of trap-rock in this country is probably the Palisades of New Jersey, although it is also found in Connecticut, Pennsylvania, and Virginia. It has a specific gravity of from 2.8 to 3.2.

Basalt. This rock does not differ materially from diabase, but

is of more recent origin. The essential minerals are augite and plagioclase feldspar with olivine. The accessories are different varieties of iron and apatite with sometimes quartz, mica, etc. Structurally it varies from the glassy to the holocrystalline. Chemically it is composed of silica 50 per cent, alumina 14, lime 10, magnesia 6, oxide of iron and manganese 12, and soda 4 per cent, with small quantities of potash, etc. In the United States it is found principally west of the Mississippi, and especially in California and Oregon. It is generally finer-grained than trap-rock. It was used very generally by the early road-builders of the old country, being carried great distances to form the surface of the roads on account of its fine wearing qualities.

Sioux Falls Stone.

This is a red quartzite or metamorphic sandstone. It contains 85 per cent of quartz. Its color is due to oxide of iron. It is said to be the hardest stone in the country. It weighs 162 lbs. per cubic foot and has a crushing strength of 28,000 lbs. per inch. On account of its hardness it is not much used for building purposes, but has been to some extent in Western cities for pavements. It wears smooth with a glassy surface.

The mineral composition of this rock was calculated from the above analysis, but nothing more than an approximate result could. be expected because the exact composition of the minerals is not known. It was supposed to be: