492 



REGINALD A. DALY 



authors writing in English on this type of intrusive body. The term 

 " sill" has also its equivalents in this sense in other languages. Defini- 

 tion II is not only contrary to general usage but suffers from special 

 imperfections due to the artificial nature of the chief criterion dis- 

 tinguishing dike and sill. (Compare Definition III of "dike.") 

 Among those imperfections is the difficulty of classifying by Definition 

 II many intercalated sheets now dipping at high angles, for with 

 them it may be impossible to say what were their original dips. 



It thus seems best to adhere to the prevailing use of the term "sill" 

 in a systematic classification of intrusive bodies. A sill may be of 



SCALE 



One .. mile 



Fig. 2 



great thickness, as illustrated on Fig. 2, but it is necessary that the 

 sheet shall hold its thickness for considerable distances along its 

 outcropping edge. 



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

 and is the result of successive injections of one kind of magma 

 along a bedding-plane in a stratified formation. 1 For a remarkable 

 illustration, see Harker. 2 



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. 3 



A section of a composite sill is illustrated in Fig. 3. 



1 See A. Geikie, Ancient Volcanoes of Great Britain, Vol. II, pp. 318 ft". ; and espe- 

 cially A. Harker, op.cit., p. 197. 

 * Op. cit., p. 239. 

 3 See Harker and Geikie, same pages as noted for multiple sills. 



