94 GROVE KARL GILBERT— DAVIS [MEM0IM [ ^ , S i £ 



EVIDENCE FOS MAGMATIC FLUIDITY 



The validity of Dana's assumption that viscosity is the cause of laccolithic doming may be 

 next examined. As above noted, this assumption takes no account of the narrow dikes above 

 the laccoliths and of the thin sheets around them, although both these classes of intrusions result 

 from the immediate entrance of the laccolithic magma into cracks and splits that appear to have 

 been produced by strains in the overlying strata, not at the first stage of sheetlike spreading 

 but later, when the domelike form of the central mass was eventually assumed. Hence the 

 magmas of the Henry Mountains laccoliths must have preserved their fluidity all through the 

 period of their intrusion in the presence of the great superincumbent pressure under which they 

 were intruded, even though the same magma might be somewhat viscous in surface eruptions. 

 Moreover, serious mechanical difficulties are involved in the assumption that laccolithic intu- 

 mescence went on pari passu with lateral spreading; for in that case it must be believed that 

 the up-arching of a small area of overlying strata by the direct upward push of the fluid magma 

 was easier than the radial extension of the magma in a thin sheet, as a result of a hydrostatic 

 change in the direction of the push from vertical through the conduit to lateral between the 

 inclosing strata. A magma must have been extremely viscous to act in this way. When it is 

 recognized that the production of a small arched dome by upward magmatic pressure in a 

 series of horizontal strata 2 miles thick is accomplished at a great mechanical disadvantage 

 as compared to the production of a flat-topped dome of much larger horizontal radius in the 

 same thickness of strata, and when it is remembered that the laccolithic magma behaved 

 essentially like a fluid in filling all available cracks and splits, it ought to be understood that this 

 same magma, when "solving the problem of least work," would have avoided the difficult 

 task of producing a small arched dome and would have chosen the easier task of extending 

 itself laterally in a thin sheet before beginning to thicken in a flat-topped dome. In other 

 words, a magma which was fluid enough to form narrow dikes and thin sheets at a late stage 

 of its intrusion, could not have been viscous enough at an early stage to persist in upward 

 pressure when a lateral escape was opened; it must have then been mobile enough to change its 

 upward pressure into a horizontal thrust at the level of laccolith formation. 



VALIDITY OF GILBERT'S VIEW 



Finally, the facts recorded by Gilbert prove that in certain cases the laccolithic magma 

 did actually begin its lateral movement by spreading horizontally between the strata in com- 

 paratively thin sheets which caused small uplift, and that the formation of flat-topped domes 

 was not undertaken until the sheets had acquired a considerable area. Of prime importance 

 here is the evidence given by the Howell laccolith, which has a depth of only 50 feet in a breadth 

 of over 2,000 feet; and yet even this sheetlike laccolith is critically distinguished from normal 

 intrusive sheets by a sudden thinning at the margin: 



In place of the tapering wedge which usually terminates intrusive sheets, there is a blunt, rounded margin, 

 and the lava scarcely diminishes in depth in approaching it. The underlying strata, locally hardened to sand- 

 stones, lie level; the overlying curve downward to join them, and between the curved strata is interleaved [ex- 

 terior to the laccolith] a curved lava-sheet. In all these characters the intrusive body is affiliated with the typical 

 laccolites, and distinguished from the typical sheets (34, 35). 



Surely if a "curved lava sheet" could have been intruded exterior to the margin of a lac- 

 colith, where the cooling of the magma must have been greatest, and yet could even there 

 have had tapering edges and a much less maximum thickness than 50 feet, the intrusion of 

 such a curved sheet contradicts the assumption that the magma of a laccolith, even at the center 

 of its supply where it must have been hottest, was so stiffly viscous as to bend up the heavy 

 overlying strata almost as much as it bent its own motion. 



Indeed, the possibility that the magma was viscous is hardly referred to in Gilbert's analysis; 

 it is always described as a fluid which obeyed the hydrostatic law, and this conception was 

 clearly not a gratuitous assumption but a well-reasoned conclusion, based on such facts and 

 inferences as have just been presented. For example, in the final summary, entitled "History 

 of the Laccolite," the explicit statement is made: 



