716 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



within the invaded formations, except along the relatively narrow openings to 

 ihe chamber where the latter has been in communication with the feeding 

 reservoir. 



On the other hand, stocks, bosses, and batholiths never show a true floor. 

 They appear to communicate directly with their respective magma reservoirs. 

 Each of these bodies shows field relations suggesting that it is a part of its 

 magma reservoir. The communication with the magmatic interior of the earth 

 is not established by narrow openings, but by a huge, downwardly enlarging 

 opening through the country-rock. In relation to the invaded formations a stock, 

 boss, or batholith is intrusive, but is subjacent rather than injected. 



How a magma reservoir is enlarged by the volume represented in the 

 amount of intrusion signalized on the contacts of stock or batholith is a matter 

 permitting as yet of no absolute certainty. In separating intrusive bodies into 

 two primary divisions, one including all injected bodies, the other including 

 subjacent bodies, a classification will do good service in emphasizing the need 

 ■of further investigation into the mechanics of intrusion. 



So far as the method of intrusion is concerned, therefore, stocks, bosses, 

 and batholiths belong to a primary division of intrusive bodies which may be 

 defined as not demonstrably due to injection. The principle is negative; it 

 leaves the method of intrusion unstated, but it brings into clear relief a principal 

 contrast subsisting between the greatest of intrusions, on the one hand, and 

 dikes, sheets, laccoliths, etc., on the other. 



The other principles of classification — viz., (b), (c), (d), and (e) — are 

 applied in the classification now to be presented in a manner sufficiently obvious 

 to need no discussion. Principle (e) is less fundamental than the others, except- 

 ing (d), and is recognized as appearing only occasionally in the scheme; the 

 major diameters of true laccoliths tend to horizontality ; the principal axis of a 

 bysmalith, neck, stock, boss, or batholith is characteristically vertical. 



It is obvious that transitional forms are to be expected among the related 

 types of the classification. These forms are not mentioned in the table, which 

 would thus become overburdened. Magmatic differentiation within dikes, sills, 

 and stocks has often produced varietal types of these bodies, but the process has 

 occurred too irregularly to permit of its furnishing a convenient criterion for 

 the^general classification. 



Injected Bodies. 



Dike. — Most geologists are agreed that dikes in stratified formations are 

 bodies always cross-cutting the bedding planes. Many geologists agree that the 

 angle of dip is i mm aterial. All agree as to the criterion 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. 



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 to be 

 proposed. 



