heights, as at fin Figure 62. These interruptions of the strata are particularly troublesome to miners working beds of coal or ore, and hence they have been called troubles, faults, or slips. The fissures are usually filled with sand, earth, and angular fragments of rocks. When the fissure extends to the surface and has considerable width, it is termed a gorge; when it is still wider, it is called a valley. 104. The thickness of strata is determined by measurements applied to their edges. If they are vertical, a measure applied horizontally to the edges gives their thickness; but if they are inclined, it is ascertained by a simple trigonometrical process. Having measured the breadth of the upturned edge, and ascertained the dip, we have the hypothenuse and angles of a right-angled triangle, from which the perpendicular side-the thickness, is easily obtained. The total thickness of the strata is various in different places. Dr. Buckland estimates the thickness of European strata at ten miles. The stratified rocks usually contain remains of plants and animals, and are then called fossiliferous.

105. The unstratified rocks exhibit no arrangement in parallel layers, are of crystalline texture, and, having undergone the action of heat, are called igneous rocks. They occur in three different positions: beneath all the stratified rocks, granite being the deepest known rock; above the stratified rocks, constituting the summits of the loftiest mountains; and thrust into the strata, as veins and dikes. The unstratified rocks are found principally in mountains, and are not widely diffused at the surface, of which they constitute not more than one-tenth, but beneath the thin crust of strata are supposed to form the great mass of the globe. They cause extensive changes in the characters

and position of the strata with which they come in contact. The unstratified rocks are entirely devoid of the remains of animal and vegetable bodies.

106. Veins are usually masses of igneous rocks injected from below into fissures in both stratified and unstratified rocks; sub-dividing as they advance, and becoming merc Fig. 63.

threads, they disappear. They are frequently chemically united to the sides of the fissures, but sometimes do not adhere to them. Their contents are sometimes influenced by the characters of the rocks through which they pass. Dikes are large veins of trap-rock, porphyry or lava, extending in some instances seventy miles, with a thickness of several yards. Dikes are nearly straight, while veins

are very tortuous. As dikes are very compact and hard, the strata through which they pass are often worn away, and they are left standing out like walls. Dikes and veins frequently intersect; in which case that which cuts through the other must be the last erupted, and hence several successive periods of eruption are proved in the granite, trap and other igneous rocks. Some veins are found entirely in

cluded in the rock, and are not traceable to any mass of similar composition but appear to have separated from the rock in which they lie. These are called veins of segregation. Veins and dikes cross the strata at various angles, and are sometimes intruded between the strata, and spread out so

as to resemble true beds; but they have no lamination. The contents of veins are exceedingly varied; indeed it is presumed that they contain all elementary substances known to chemists. They are divided into two classes the metalliferous and non-metalliferous. The contents of dikes are much more limited in kind and uniform in character.

107. A concretionary structure on a large scale is occasionally seen in igneous rocks, but a more interesting structure exhibited by them is that which produces regular columns, varying in size from an inch in diameter to several feet; in length, from one to three hundred feet; and

Fig. 65.

Jointed Columns.

in number of sides, from three to twelve. They are so accurately adjusted to each other that no space intervenes

between them, and fre

quently consist of joints with alternate convex and concave surfaces: they are usually straight, but sometimes highly curved.

108. Mr. Lyell distinguishes the igneous rocks into two classes the volcanic, and the plutonic. The volcanic are those which have been produced at or near the surface of the earth, as are the lavas of volcanoes of the present period; but similar rocks have been poured out upon the land or the bed of the ocean, and have been injected into fissures near the surface, at many different epochs. The plutonic rocks appear to have been formed under enormous pressure at great depths in the earth. They differ from

the volcanic in being more highly crystalline, and free from the pores, or cellular cavities which characterize the volcanic. The granites and porphyries belong to this class.

109. There is another class of rocks, which partake of the characters of both aqueous and igneous rocks. These lie upon the plutonic rocks, are highly crystalline in structure, are destitute of organic remains, and yet are divided into beds precisely like the sedimentary formations in form and arrangement. These strata appear to have been deposited in water in the usual way, and then to have been subjected to such a degree of subterranean heat as to assume a new texture. In some instances a portion of a stratum has exchanged its earthy for a crystalline texture through a distance of a quarter of a mile from its contact with granite, and has had all traces of its organic remains obliterated, while the remainder of the stratum retains all the characteristics of its sedimentary origin. Thus dark colored limestones filled with shells and corals have been converted into white statuary marble, and clays into slates, or schists. These altered rocks are called metamorphic.

110. All rocks, whether stratified or not, are divided into masses of determinate figures, by natural fissures traversing them in straight lines, and forming planes of variable width. These fissures are called joints. Their faces are usually smoother and more regular than the planes of stratification, to which they are vertical, thus dividing the rock into cubical or prismatic blocks. Some joints are more open, regular and continuous than others, passing through several alternations of strata, dividing them from top to bottom, sometimes completely arresting the cross joints. These are called master-joints. The constancy of