REPORT OF THE CHIEF ASTRONOMER 721 



SESSIONAL PAPER No. 25a 



sides. Sometimes they are seen to have pushed the surrounding rocks aside. 



In other places they seem to occupy the place of these rocks through which, 



as it were, an opening has been punched for the reception of the intrusive 



material. ... In true bosses, unlike sills or laccolites, we do not get 



to any bottom on which the eruptive material rests.' 



He makes ' stock ' and ' boss ' synonymous. 



In English and German-speaking countries ' boss ' and ' stock ' are almost 

 invariably regarded as synonymous, but the latter term has much the greater, 

 vogue. The general connotation of the word ' boss ' seems to warrant the restric- 

 tion 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 intrusions 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 similar to that between ' multiple ' and ' composite ' stocks. 



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 irre- 

 gular and not determined by planes of bedding or other structures in the coun- 

 try-rocks. It has no visible floor. 



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 originally derived from 

 two or more kinds of magma. 



Magmatic differentiation or other influences may render heterogeneous the 

 material composing a simple stock, or each member of either a multiple or a 

 composite stock. 



Batholith. — Suess has finally stated the definition of 'batholith' in terms of a. 

 theory of intrusion which is at present in discussion. His definition may be freely 

 translated thus : ' A batholith is a stock-shaped or shield-shaped mass intruded 

 as the result of fusion of older formations (orig. Durchschmelzungsmasse) . 

 On the removal of its rock-cover and on continued denudation, this mass either 

 holds its diameter or grows broader to unknown depths (orig. Us in die ewige 

 Teufe* The name was invented to describe those largest of all intrusions, 

 generally granitic, which are characteristically found in great mountain ranges; 

 including, thus, ' central granites,' ' intrusive mountain-cores.' and ' Fuss- 



* Sitzungsberichte der Wiener Akademie, Vol. 104, 1895, p. 52. 



