488 REGINALD A. DALY 



DEFINITION OF TERMS DESCRIPTIVE OF INTRUSIVE BODIES 



Dike. — The best established of all the terms used in the foregoing 

 lists is "dike;" yet the variation in even recent definitions of it is 

 apparent in such examples as the following: 



I. G. K. Gilbert: 1 



Dikes differ from sheets in that they intersect the sedimentary strata at greater 

 or less angles, occupying fissures produced by the rupture of the strata. 



II. G. P. Merrill: 2 



A dike: "an eruptive mass of varying width included between well-defined 

 walls, and occupying a fissure or fault in previously consolidated rocks. Such 

 are inclined at all angles with the horizon, and are usually of very moderate width, 

 but may extend for miles." 



III. T. A. Jaggar;3 



A dike is an elongate intrusive igneous body occupying a fissure in any soit 

 of rock, the walls of which at the time of intrusion were vertical or, if inclined, 

 at angles nearer the vertical than the horizontal. A dike must have longitudinal 

 extension much greater than its breadth, but may vary in thickness from an inch 

 to several hundred feet. A dike may be irregular or may follow a sinuous course; 

 it may be intruded between the beds of vertical or steeply inclined sediments; 

 it frequently follows joint surfaces and has smooth and plane bounding walls. 

 It must be noted that a flat igneous mass intruded between horizontal or nearly 

 horizontal strata, and subsequently upturned with them to a vertical position, 

 is not a dike, but a sill. 



IV. Sir Archibald Geikie: 4 



Dykes are veins of eruptive rock, filling vertical or highly inclined fissures, 

 and are so named on account of their resemblance to walls (Scotice, dykes). 



V. T. C. Chamberlin and R. D. Salisbury: 5 



When lava is forced into crevices or rises to the surface through fissures, 

 and the residual portion solidifies in them, it gives rise to dikes. 



All the foregoing definitions agree in ascribing to a dike the char- 

 acteristic form of a fissure-filling. Gilbert, in the quotation; Geikie, 

 and Chamberlin and Salisbury, in their respective contexts, expressly 

 exclude from the category of dikes all sheetlike intrusions thrust 



1 Geology of the Henry Mountains (1877), p. 20. 



2 Rocks, Rock Weathering and Soils (1897), p. 50. 



3 Twenty-first Annual Report of the U. S. Geological Survey, Part 3 (1901), p. 172. 



4 Text-book oj Geology (4th ed., 1903), Vol. II, p. 743. 

 s Geology (1904), Vol. I, p. 476. 



