274 University of California Publications in Geology [Vol. 13 



The heat given off from the mass must have so expanded the 

 adjacent rocks as to set up severe strains in them ; but if the acces- 

 sions of magma were as I have indicated in a previous paragraph, 

 these heat effects could scarcely be discriminated from those which 

 accompanied its contraction. In considering the mechanical effects 

 of this magmatic heat it would be the logical thing to divide them 

 on a time basis separated by the instant when expansion had reached 

 its maximum. It is generally admitted that the heat conductivity of 

 rock is very low, and that at the time of its injection an igneous mass 

 does not impart its heat to a great thickness of wall-rock surrounding 

 it. Assuming a depth of 24,000 feet, and a magma 1800° F. above 

 the temperature of its walls. Daly 5 calculates that in average rock at 

 a distance from the magma of 400 feet no heat would be felt in the 

 course of 16 years, and that in 100 years the temperature would have 

 been raised only 283° F. In view of this it would be difficult to imag- 

 ine that the vertical expansion of the foot wall of the diabase sheet at 

 Cobalt could have amounted to many feet during the entire period of 

 injection and solidification of the magma. However, the expansion 

 tendency of this rock would not have been only upward ; it would have 

 been also lateral ; and the summation of that tendency over the lateral 

 dimensions of the sheet must have totalled to a force of considerable 

 magnitude. The escape of this increased volume of rock, inhibited 

 laterally, must have been upward ; but its upward movement would 

 have been caused by reaction against the force of its lateral expansion, 

 taking the form either of reverse faulting or of folding. 



Since the margins of the sheet are chilled, and no assimilation of 

 its walls occurred, it must have made room for itself by mechanical 

 displacement, probably entering as a thin wedge, and splitting its 

 own path before it. This procedure is evidenced by the indifferent 

 transgression of the surface of injection across beds and contacts. 

 It is very significant that the original undulations of this surface con- 

 form to the major structural axes of the region, as if following a zone 

 of weakness and shearing due to incipient folding on those axes. If 

 it be assumed that the magma ascended from the depths along one of 

 these axes, advancing laterally with a fairly even front from that 

 starting point, then perhaps the lateral pressure resulting from the 

 expansion of the underlying rocks would have advanced before its 

 wedge-like front parallel with its initial alignment causing incipient 

 folding parallel with the structural axes, which thus produced the zone 

 of weakness invaded by the magma. This method of origin of the 



s Igneous rocks and their origin, p. 198. 



