GENERAL GEOLOGICAL FACTS AND PRINCIPLES. U 



A monocline is a terrace-like dropping of a bed without changing 

 the direction of the dip. There is usually a zone, more or less 

 shattered, along the folded portion, and such a zone may become 

 a storage receptacle. Monoclines of a gentle character in Ohio, 

 which have been detected by Orton in studies of natural gas, have 

 been called "arrested anticlines." An anticline is a convex fold 

 with opposing dips on its sides, while a syncline is a concave fold 

 with the dips on its sides coming together. We speak of the axis 

 of a fold, and this marks the general direction of the crest or 

 trough. The axis is seldom straight for any great distance. Folds 

 are often broken and faulted across the strike of their axes, and 

 this causes what is called a "pitch "of the axes and makes the 

 original dips run diagonally down on the final one. Folds are the 

 primary cause of the phenomena of dip and strike. Horizontal 

 beds have neither. A dome-like elevation of beds, with dips radi- 

 ating in every direction from its summit, is called a quaquaversal, 

 but it is a rare thing. An anticline or syncline w r ith equal dips on 

 opposite sides of its axes is called a normal fold. If the dip is 

 steeper on one side than on the other, it is an overthrown fold ; if 

 the sides are crushed together, it is a collapsed or sigmoid fold. 



Igneous rocks are in the form of sheets (the term "bed" should 

 be restricted to sedimentary rocks), knobs or bosses, necks, lacco- 

 lites, and dikes. A sheet is the form naturally assumed by surface 

 flows, and by an igneous mass which has been intruded between 

 beds. It has relatively great-length and breadth as compared with 

 its thickness, and coincides with its walls in dip and strike. A 

 knob, or boss, is an irregular mass, of approximately equal length 

 and breadth, which may be related in any way to the position of 

 its walls. Such masses are often left projecting by erosion. A 

 neck is the filled conduit of a volcano, which sometimes remains 

 after the overlying material has been denuded. A laccolite is a 

 lenticular sheet which has spread between beds radially from 

 its conduit, and thus has never reached the surface, unless re- 

 vealed by subsequent erosion. A dike is a relatively long and 

 narrow body of igneous rock which has been intruded in a fissure. 

 It is analogous to a vein, but the term " vein " ought not to be ap- 

 plied to an undoubtedly igneous rock. Some granitic mixtures, 

 however, of quartz, feldspar, and mica, leave us yet in uncertainty 

 as to whether they are dikes or veins. (See Example 56.) From 

 the above it will be seen at once that bosses, knobs, and necks may 

 be practically indistinguishable. 



