502 REGINALD A. DALY 



In English- and German-speaking countries "boss" and "stock" 

 are almost invariably regarded as synonymous, but the latter term 

 has much the greater vogue. 1 The general connotation of the word 

 "boss" seems to warrant the restriction of its meaning so as to include 

 only those stocks which have circular or subcircular ground plans on 

 the surface of exposure. The word has been used to denote intru- 

 sions of the sort up to all diameters from a few hundred feet to several 

 miles. 



Bosses are "simple" when composed of material intruded in but 

 one period; they are "multiple" or "composite" when composed 

 of material intruded at two or more distinct periods of irruption. 

 The distinction between the latter types is the same as between 

 "multiple" and "composite" stocks. 



An illustration of a simple boss is given in Fig. 8. 



Stock. — Prevailing usage has fixed the meaning of "stock" as 

 essentially equivalent to Geikie's definition of "boss." A stock is 

 an intrusive body, but is not as clearly injected as is the case with 

 a dike, sill, or laccolith. A stock more or less conspicuously cuts 

 across the structures of the invaded formations; its contacts are, in 

 general, either vertical or highly inclined; its shape is irregular and not 

 determined by planes of bedding or other structures in the country- 

 rocks. It has no visible floor. Van Hise regards a stock as char- 

 acteristically smaller than a boss, 2 but the present writer has found 

 that general usage does not support that distinction. 



Simple stocks are composed of material intruded in one period 

 of irruption. 



A multiple stock is composed of material demonstrably intruded 

 in two or more periods of irruption, the material having been derived 

 from the same kind of magma. 



A composite stock is composed of materials demonstrably intruded 

 in two or more periods of irruption, the materials having been origi- 

 nally derived from two or more kinds of magma. (Fig. 8.) 



Magmatic differentiation or other influences may render hetero- 

 geneous the material composing a simple stock, or each member of 

 either a multiple or a composite stock. 



1 Cf. Zirkel, Lehrbuch der Petrographie (1893), Vol. I, p. 539. 



2 Monograph No. 47, U. S. Geological Survey (1904), p. 711. 



