CLASSIFICATION OF IGNEOUS INTRUSIVE BODIES 489 



into the bedding-planes of stratified formations, whatever is or has 

 been the angle of dip. The same usage appears in the writings of 

 most field geologists, whether English-speaking or not. Geikie 

 emphasizes the generality (necessity?) of high angles of dip in true 

 dikes. Jaggar makes the angle of dip at the time of intrusion the 

 principal criterion in distinguishing dikes from sills, and advocates 

 a definition of both dike and sill which is not followed by the great 

 majority of geologists. J. D. Dana 1 states that dikes may vary 

 "in position from vertical to horizontal," and it is seen that Gilbert's 

 and Merrill's definitions agree with that usage of the term. Since 

 most dikes are nearly or quite vertical, the difficulty of classifying 

 those eruptive fissure-fillings which lie nearer horizontal than vertical 

 planes has not often been mentioned in geological literature. 



Most geologists are thus agreed that dikes in stratified formations 

 are bodies always cross-cutting the bedding planes. Many geologists 

 agree that the angle of dip is immaterial. All agree as to the cri- 

 terion of form, namely, that of a fissure-filling narrow in proportion 

 to its length and bounded by parallel or nearly parallel walls of 

 country rock. 



According to the commonest and best usage, an igneous dike (a) 

 is an injected body, (b) has nearly or quite parallel walls, (c) is narrow 

 in proportion to its outcropping edge, (d) cuts across the bedding 

 when the invaded formation is stratified, and (e) has any angle of dip. 



When stratification and cleavage or schistosity are not coincident, 

 such an intrusive body is generally called a dike, even though it 

 follows the planes of cleavage or schistosity. This usage will be 

 adopted in the classification proposed in this paper. 



Multiple dikes are compound intrusions of dike form, due to 

 successive injections of homogeneous material on the same fissure. 2 

 For illustrations, see Harker, Tertiary Igneous Rocks 0} Skye, pp. 

 296-304; A. Geikie, Ancient Volcanoes, Vol.. II, p. 417. 



Composite dikes are compound intrusions of dike form, due to 

 successive injections of different materials into the same fissure. 3 



1 Manual of Geology, 4th ed., 1895, p. 298. 



2 Geikie, Text-book of Geology, Vol. II, p. 746. 



3 J. W. Judd, Quarterly Journal of the Geological Society, London, Vol. XLIX 

 (1893), p. 536; A. Harker, Tertiary Igneous Rocks of Skye (" Memoirs of the Geological 

 Survey of Great Britain," 1904), p. 197. 



