764 



DYNAMICAL GEOLOGY. 



products of unknown volcanoes, high up in the mountains. The baked rocks, besides 

 giving their red tints to the country, resist erosion, as Mr. Allen states, and so protect 

 the hills from denudation, and become prominent features of the region. 



The production of the metamorphic results by the heat of erupted 

 rocks, and the extent of the region affected, has depended chiefly on 

 the presence of moisture for conveying and utilizing the heat. Near 

 New Haven, Conn., the sandstone walls of a dike crumble in some 

 places into small chips, apparently because of the want of moisture there 

 at the time of the eruption, and in others, the rock, although a coarse 

 conglomerate, is very firmly consolidated. The presence of steam is 

 often indicated by remains of the tubular channels through which it 

 rushed, their walls bleached and penetrated with chlorite ; and the 

 chlorite, in some places near by, is fibrous in structure and spangled 

 with minute but perfect crystals of hematite. 



A region invaded by trap eruptions is often also, as a consequent or 

 concurrent fact, a region of steaming fissures and of hot springs, con- 

 veying the heated moisture widely through the strata of the region ; 

 and thus the sand-beds of the same Mesozoic formations in the Con- 

 necticut Valley were generally reddened as well as consolidated — 

 oxydation of iron, when taking place through the agency of hot wa- 

 ters, producing the anhydrous sesquioxyd of iron or the red oxyd. 



These examples of alteration illustrate not only local, but also 

 regional metamorphism, for the minerals formed are among those that 

 figure extensively in metamorphic rocks. Chlorite, garnet, tourma- 

 line, are among the most common of such minerals ; and if these and 

 other species can be made under the rather rapid and coarse conditions 

 afforded by the eruption of an igneous rock, much more complete 

 should be the results of slow-working metamorphism. It is observed, 

 also, that these minerals are made by selecting and combining the 

 needed elements; it is not mere crystallization. The iron of the 

 epidote, chlorite, garnet, tourmaline, must be the iron that gives the 

 red color almost everywhere else to the sandstone. The tourmaline 

 crystals seem to show that marine waters (or, perhaps, borate springs, 

 made earlier from the ocean's waters) supplied the boracic acid needed 

 in their constitution. The hematite crystals (Fe 2 O s ) were probably 

 derived from the oxydation of the magnetite grains (Fe 3 4 ) of the 

 sand-beds. The quartz crystals were made out of silica taken from 

 the siliceous minerals (feldspar, etc.) that were decomposed by the 

 steam to furnish material for the new crystallizations ; and the heat, as 

 far as it reached through the sand-beds, even if of low degree, made 

 in the same way the siliceous solutions that produced the consolidation 

 of the sand-beds. 



