Structural geology . sly been under conditions notthose of hydration and carbonation, or katamorphism in have suggested that gneisses may form only in places wherethe agents of hydration and carbonation are lacking, and thatwhere these agents were present, the change is more toward theschist type. The terms schist and gneiss have been used as representingtwo contrasting types of rocks. It is of course to be recognizedthat there are complete gradations between schist and gneiss; thatit probably follows therefore that there are many conditions oforigin of the schists and gneisses from
Structural geology . sly been under conditions notthose of hydration and carbonation, or katamorphism in have suggested that gneisses may form only in places wherethe agents of hydration and carbonation are lacking, and thatwhere these agents were present, the change is more toward theschist type. The terms schist and gneiss have been used as representingtwo contrasting types of rocks. It is of course to be recognizedthat there are complete gradations between schist and gneiss; thatit probably follows therefore that there are many conditions oforigin of the schists and gneisses from igneous rocks intermediatebetween those described. STRUCTURES COMMON TO BOTH FRACTUREAND FLOW FOLDS ELEMENTS OF FOLDS The elements of a simple fold are indicated in the followingdiagram (Fig. 48) taken from Willis. The attitude of a rock bed is described in terms of strike and is the direction of line of intersection of the bed with thehorizontal; dip is the angle between the bed and the horizontal, PITCH. Fig. 48. Parts of folds. After Willis. measured at right angles to the strike. Folds are usually deter-mined by the correlation of strike and dip observations. The axial plane of a fold intersects the crest of trough in such amanner that the limbs or sides of the fold are more or less symmet-rically arranged with reference to it. The intersection of theaxial plane with the crest or trough of a fold is the axial line, axis, 104 ELEMENTS OF FOLDS 105 crest line, or trough line. The pitch of the fold is the inclination ofthe axial line to the horizontal. It is merely a special case of diptaken along the axis. Strike and pitch are never strictly parallel, although if the pitchis slight, they may be nearly so. A simple fold is a single bend or curve without minor crenula-tions. A composite fold is the simple fold with minor crenulationssuperposed on it. A complex fold is one which is cross folded,that is, one of which the axial line is folded. As defined by VanHise, co
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Keywords: ., bookcentury1900, bookdecade1910, bookpublishernewyo, bookyear1913
