i6 JOURNAL OF GEOLOGY— SUPPLEMENT 



As the more specific descriptions given later will show, sills 

 and laccoliths of basic magma very commonly give evidence of this 

 process when of sufficient thickness, which apparently must, as a 

 rule, be as much as 500 to 1,000 feet or more. Much thinner sills 

 sometimes show it^ and much thicker sills often fail to show it, but 

 this is nothing more than is to be expected. A small sill which is 

 one of a large number intruded at approximately the same time 

 into the beds of a sedimentary series will cool more slowly than a 

 sill of equal size which is practically alone in such a series. Many 

 thick basic sills or laccoliths are possibly not the result of instan- 

 taneous intrusion, the spreading apart of the strata being slowly 

 accomplished so that the thickness of the sill may not have been 

 increased much faster than the thickness of the layers of frozen 

 magma formed on roof and floor. In such a case no differentiation 

 of the type referred to is to be expected. Repeated violent shocks, 

 localized escape of gases, or anything which might cause a streaming 

 of the liquid magma may also be expected to prevent differentiation. 

 Finally, bodies of great size in which this type of differentiation has 

 occurred are especially liable, because of their slow crystallization, to 

 be still partly liquid at a time of important later disturbances, 

 and these are likely to obscure the simple density arrangement of 

 differentiates and especially to obliterate the evidences of gradual 

 transition of one type into another. Thus tongues of the still 

 liquid portion may be injected into fissures and cracks formed in 

 the crystallized portion and give the impression of having arrived 

 as the result of a separate and distinct principal act of intrusion 

 when really a product of differentiation practically in place. 



Important effects due to the sinking of crystals are hardly to be 

 expected in sills of salic magma. Such magma depends for its 

 fluidity on its ability to retain volatile constituents, whereas the 

 basic magmas are comparatively fluid in virtue of the nature of the 

 silicates themselves. The intrusion of a salic magma as a sill- 

 like body, exposing a great total surface in more or less porous 

 rocks, favors the escape of much of its volatile material. The 



' The very clear example of sinking of pyroxene crystals in a sill only 30 feet thick 

 described by Iddings from Yellowstone Park must be regarded as a very exceptional 

 case {U.S. Geol. Survey, Mon. XXXII, Part 2 [1889], pp. 82-84.) 



