

METAMORPHISM. 763 



— without marked metamorphism below. Professor Geikie remarks 

 that in the South Wales coal-field the Carboniferous limestone, al- 

 though covered by other rocks to a depth of 10,000 to 12,000 feet, is 

 unaltered, while the rocks of the Central Highlands of Scotland are 

 intensely altered, though they were not at the time of metamorphism 

 covered by more than 5,000 feet of strata. 



5. Local Metamorphism. 



Local metamorphism has often occurred in the walls or vicinity of 

 dikes of eruptive rocks, and in regions of hot springs. For example : 

 " Crystallizations of epidote, tourmaline, garnet, chlorite, quartz, hem- 

 atite and magnetite, besides various zeolites, occur in the Triassico- 

 Jurassic red sandstone of the Atlantic border of North America, in 

 the vicinity of the trap-dikes which intersect it, that were produced 

 through the agency of the heat the trap had when ejected. The gar- 

 nets occur at Mill Rock, New Haven, Conn., in the sandstone within 

 a few yards of the trap, and also in rifts in the trap near its walls ; 

 and those in the latter are yellow topazolites of great beauty, though 

 small. At Rocky Hill, N. J., according to H. D. Rogers, the 

 "baking" effects of a trap-dike are distinct for a fourth of a mile 

 from the dike ; and, fifty feet off, a thin bed contains " kernels of pure 

 epidote," and cavities that are " studded with crystals of tourmaline." 

 At one place the latter crystals are half an inch in diameter. 



The sandstone, where containing these minerals, has generally lost its 

 usual red color, and become grayish-white to greenish, the green color 

 coming from the chlorite generated by the heat. The same trap dikes 

 also intersect chlorite slate to the west of New Haven, Conn., and 

 here the rock is changed from a dull green to black, owing to the pro- 

 duction of some magnetite, in accordance with the common blow-pipe 

 result when an iron-bearing silicate is heated. 



A trap dike intersecting the clayey layers, sandstones, and coal-beds of the island of 

 Nobby, New South Wales, has baked the clayey layers to a flint-like rock (as described 

 in the author's Expl. Exped. Report), to a distance of two hundred yards from the 

 dike, the whole length of the island; the baking effect must have continued much 

 farther. 



Baking effects, and sometimes crystallizations, have been occasioned by the burning 

 of coal-beds. 



Mr. J. A. Allen states that the burning of the coal-beds of the Lignitic Tertiary of 

 Dakota and Montana — how ignited is not known — has changed clays to hard and 

 sometimes porcelain-like rocks, usually reddening them, and also to beds of a half- 

 fused cellular or scoriaceous and pumice-like character, looking like the products of a 

 volcano. One of the regions thus burnt over, on the Little Missouri, is twenty to thirty 

 miles broad by two hundred miles in length. Others occur in the Yellowstone at the 

 mouth of Powder River, and along the latter stream; about the sources of Tongue 

 River, within a few miles of the Big Horn Mountains, and on the north fork of the 

 Cheyenne River, as observed by Hayden. Fragments of pumice have been found on the 

 Missouri as far south as Fort Pierre, and the early explorers supposed them to be the 



