Handbook of Paleontology 65 



with distinctly arched roof. They, too, may be inter- 

 formational. Sheets and laccoliths are usually concord- 

 ant with the bedding, but they may break across the 

 bedding locally. Dikes are sheets of igneous material not 

 concordant with the bedding, and are the result of the 

 simple filling of fissures in rock masses by molten magma 

 from below. They not only cut across the stratification 

 planes of sedimentary rocks but also pass through other 

 igneous rocks or metamorphic rocks. Dikes may vary 

 from a fraction of an inch to half a mile or so in thick- 

 ness, and an individual dike may thin out or pinch and 

 swell along its length. An intrusive mass, nearly cylin- 

 drical, with vertical or steeply inclined axis, is called a 

 neck. Batholith is the term applied to those huge irregu- 

 lar masses of igneous rock that have no base and are dis- 

 cordant in their relations with the rocks they invade. 

 They are the deep-seated or abyssal rocks, exposed by 

 erosion, which are found in the oldest areas of the earth's 

 crust, such as our Adirondack mountains. A small 

 batholith is termed a stock, which becomes a boss if it 

 has a circular ground plan. Stocks and bosses may be 

 only upward extensions of concealed batholiths. A 

 batholith may be exposed over thousands of square miles 

 and is arbitrarily considered as covering more than 40 

 square miles. In extrusive rocks the extrusion may be a 

 quiet flow or explosive. The quiet extrusions may be 

 fissure eruptions in which the molten rock rises to the 

 surface and pours out as a flow of lava, forming an 

 extrusive sheet, or central eruptions in which the outflow 

 is from a tubular opening or pipe. In the former, the 

 solidification of material in the fissure, when outflow has 

 ceased, forms a dike or sill, according to its relations ; in 

 the latter a volcanic neck is formed, such as Stark's 



