

720 DYNAMICAL GEOLOGY. 



enough to sustain volcanic action. In Eastern North America at epochs 

 when there was the greatest amount of friction and crushing, — those 

 of the making of the Green Mountains, and the Appalachians, — no vol- 

 canoes were made and very little took place in the way of eruptions 

 through fissures ; while at an epoch in Mesozoic time, when little fric- 

 tion and no crushing occurred, judging from the slight change of posi- 

 tion in the rocks, extensive doleritic eruptions occurred at intervals for 

 1,000 miles along the Atlantic border. (See p. 417.) The facts from 

 the region between the summit of the Rocky Mountains and the Pa- 

 cific are of similar import. Igneous rocks have a close resemblance to 

 granite, dioryte, granulyte and other crystalline kinds, and hence may 

 have proceeded from the fusion of older kinds. But these older kinds 

 derived their material from an older source, and originally from the 

 fused material of the globe ; so that the proof of such an origin by 

 Refusion is not established beyond a doubt. 



2. DILATATION AND CONTRACTION. 



1. Amount of Dilatation. — The amount of dilatation of rocks is 

 mostly between 1 and 10 millionths for 1° F. ; and one millionth cor- 

 responds to 1-67 thousandths of an inch for 100 feet. 



Colonel Totten, having observed that the stones of the coping of a wall became loos- 

 ened from some cause, made experiments, in 1830 to 1833, on effects of change of tem- 

 perature. He found that an inch of fine-grained granite expands for 1° F. -000004825; 

 of the granular limestone of Sing Sing, N. Y., -000005668 ; of red sandstone, from 

 Portland, Conn., -000009632. See also Adie's results in Trans. R. Soc, Edinburgh, 

 xiii., and Q. J. G. Soc, 1847. Pfaff found for the dilatation between the ordinary tem- 

 perature and red heat (about 1750° F.) of granite from the Fichtelgebirge, 0-0168; for 

 porphyry from the Tyrol, 0-0127; and for basalt, of Auvergne, 0120. 



Bunker Hill Monument, a hollow obelisk, two hundred and twenty-one feet high and 

 thirty feet square at base (made of granite blocks), swings to one side and the other, 

 with the progress of the sun during a sunny day — a pendulum suspended from the 

 centre of the top describing an irregular ellipse nearly half an inch in its greatest diam- 

 eter (Horsford). 



2. The Heat from an External Source. — The sun is producing 

 somewhere, at all times, alternations of temperature, and thereby 

 change of size and position ; and the same effect comes from changes 

 of temperature, whatever the source. The cause is universal in its 

 action. Such a cause, working day after day about rocky peaks and 

 precipices, causing each day some displacement, may end in degrada- 

 tions of geological importance. Besides shifting the positions of 

 masses of rock, it causes expansion and contraction of thin portions of 

 the exterior of rocks, and in some kinds leads to a peeling off of thin 

 layers, as observed by Shaler, or to the opening of delicate fractures 

 that give access to air and moisture for chemical work. 



