R. A. Daly — Mechanics of Igneous Intrusion. 115 



tain typical trials may be given in full. " A bar of fine bard 

 liglit brown sandstone, sqnare-dressed to a smooth surface, 

 measuring between two fixed points 7*34:6 inches long at 61° 

 F., and roughly speaking If inch by 1 inch scantling, was 

 placed with its ends resting fiatways on two supports. The 

 flame of the blowpipe w^as brought to bear upon the upper sur- 

 face until it expanded to 7'385 inches. The stone was hot 

 enough to melt solder but not lead — perhaps 500° F. on the top 

 surface. As the blowpipe played upon the stone it became, in 

 places immediately under the flame, momentarily heated on 

 the surface to a bright red heat, and the sharp edges were 

 burnt ofl ; the stone became when cool of a darker brown, but 

 apparently was not otherwise altered by heat. The stone 

 arched upwards against gravity in a most remarkable manner 

 to the extent of -g^^- of an inch. A second heating brought it 

 up to not less than y^-g- inch, at which it took a permanent set."* 

 Bars of Sicilian white marble and of oolitic marble, about 14*5 

 inches in length, were similarly arched y^g- inch and permanently 

 distorted. A square slab of Sicilian marble 12 inches on the 

 side and f inch thick was placed upon tw^o bars of slate. " It 

 was heated with a blowpipe on the upper surface. The flame 

 was rather small, and was kept mostly near the centre w^ithin 

 the circle of dotted lines (shown in Reade's figure), but was 

 also moved about. The slab became very slightly convex. 



Eventually it cracked The crack was the result of the 



circumferential and radial expansion of the centre part of the 

 slab, within the dotted lines, and shows that a considerable 

 bursting strain was developed. "f 



The conditions of dilferential heating in these experiments 

 are analogous to those found at intrusive contacts. The w^arp- 

 ing of the shell of country-rock immediately adjacent to the 

 molar contact must be of a higher ()rder than in the case of the 

 stone slabs because of the fact that the shell is nowhere free to 

 expand in the plane of the heated surface. The warping would 

 not be interfered wdth by the pressure of the magma if the lat- 

 ter had access through opening fissures to the outer, cooler 

 surface of the shell. Such fissures would be formed along 

 original planes of fission roughly parallel to the contact, or 

 along shearing planes produced in the same position by 

 virtue of the differential expansion itself. With the intrusion 

 of this apophysal material and the more thorough heating of 

 the warped shell, expansion would be intensified, and, as has 

 been seen, a force would be developed vastly greater than the 

 strength of the rock could withstand. The bending or shear- 

 ing shell must finally collapse and become shattered into separ- 



* Origin of Mountain Eanges, London, 1886, p. 22. 

 t Ibid, p. 23. 



