REPORT OF THE CHIEF ASTRONOMER lit 



'SESSIONAL PAPER No. 25a 



Multiple dikes are compound intrusions of dike form, due to successive 

 injections of homogeneous material on the same fissure.* 



Composite dikes are compound intrusions of dike form, due to successive 

 injections of different materials into the same fissure.f Judd recognized a 

 second class of composite dikes as those ' in which a differentiation has gone on 

 in the material that has filled the dike/ These may be distinguished as 

 ' differentiated dikes.' The nomenclature given in the above definitions brings 

 out the analogy with ' multiple ' and 'composite ' sills and laccoliths — types 

 .already well named and established. 



A dike network is a reticulate group of dikes simultaneously injected. For 

 illustration see Bulletin No. 209, TJ. S. Geological Survey, 1903, section on 

 Plate 7. 



In the preliminary paper on the classification of igneous masses a place 

 was accorded to ' intrusive veins ' and ' contemporaneous veins,' as defined in 

 Geikie's Text-book of Geology. Later discussion with friends, particularly 

 Professors R. W. Brock and Joseph Barrell, has led the writer to exclude these 

 terms from the adopted classification. Messrs. Brock and Barrell agree that 

 it would be better to restrict the name ' vein ' to a mass of material crystallized 

 from passing solutions; their point seems to be well taken. 



Apophyses or tongues are dikes which, either directly or by inference from 

 field relations, can be traced to larger intrusive bodies as the source of magmatic 

 supply. 



Intrusive sheet. — This familiar expression has generally been defined as 

 equivalent in meaning to ' sill.' It may well be extended to cover the case of 

 -an igneous layer injected on a plane of unconformity in stratified formations, 

 when the igneous layer is thus sensibly parallel to the bedding planes of one 

 of the stratified formations. This type, for lack of a better term, may be called 

 an interformational sheet. For illustration of such a sheet on a colossal scale, 

 see 'Map of Northern Nickel Range,' Sudbury District, Ontario, by A. P. 

 Coleman. 



A sill is a sheet of igneous material which has been injected into a sedimen- 

 tary series and has solidified there, so as to appear more or less regularly 

 intercalated between the strata. (A. Geikie). 



A multiple sill is a compound intrusion of sill form and relations, and is 

 i,he result of successive injections of one kind of magma along a bedding plane 

 •in a stratified formation. 



A composite sill is a compound intrusion of sill form and relations, and is 

 the result of successive injections of more than one kind of magma along a 

 bedding plane in a stratified formation. (See Fig. 25, p. 396.) 



Laccolith.- — Those who have made actual researches among laccoliths, and 

 have preserved the term ' laccolith ' with the original meaning of Gilbert's 



* Geikie, Text-book of Geology, Vol. 2, p. 746. For illustrations, see Harker, Ter- 

 tiary Igneous Rocks of Skye, pp. 296-304 ; A. Geikie, Ancient Volcanoes, Vol. 2, p. 417. 



t J. W. Judd, Quarterly Journal of the Geological Society, London, Vol. 49, 1893, 

 p. 536 ; A. Harker, Tertiary Igneous Rocks of Skye (' Memoirs of the Geological Sur- 

 vey of Great Britain,' 1904), p. 197. 



