Study of the Neio York Obelisk as a Decayed Boulder. 95 



of hammer or thrust of cane, tumbles at once into fragments, a 

 fascinating but most perplexing problem is offered in trying to 

 unravel the vicissitudes of its past history. Through the work of 

 Dolomieu, T. Sterry Hunt, and others, we have caught some glimp- 

 ses of its quiet youth, when, locked up within the original rock- 

 stratum, its rounded form was first slowly etched out by the under- 

 ground gnomes — the forces of subterranean disintegration and 

 chemical decay. 



Then followed the stirring experiences of its middle age, when, 

 in our latitude, torn out by torrent or by the continental glacier 

 from its softened bed, it was rasped by partly decayed and angular 

 gravel, hurled down deep fissures, crushed under the enormous 

 weight of thousands of feet of ice, jammed against other boulders, 

 ground down over the rocky glacier-bottom, and at times rolled 

 over and over in the rush of a glacier-river. 



At last came old age, when, stranded upon the surface of the 

 land, it was drenched by rains or melting snow, repeatedly surface- 

 dried by intense heat of summer's sun, even roasted at times by 

 passing forest-fires, frozen and thawed again and again, and soaked 

 in organic acids from soil or swamp, until completely changed in 

 molecular arrangement, and partly in material, through and through. 



Within, by absorption of oxygen and water, and consequent 

 production of new salts and combinations of increased molecular 

 volume, the entire aggregate of mineral crystals remained locked 

 in intense strain, the relief attained by partial closing of old joint- 

 planes having been offset by development, through such minerals 

 as the feldspars, of innumerable fine clefts and spongy vacuoles. 



Without, by the insinuation of water and thrusting force of frost- 

 crystals, the co-adherence of the grains was loosened, the inner 

 strain largely relieved, and the outer part of the boulder expanded 

 in a series of coats, successively softer, more porous and swollen 

 toward the exterior. 



So at last the successive crusts have tended to exfoliate and fall 

 away, until many an aged boulder has crumbled to fragments and 

 dust, with its story forever untold. 



In the case of some particular boulder, the student may often 

 make out part of this history, its original site and source, its glacial 

 experience, the distance of its transport, etc. ; but as to the exact 

 agents of decay, their relative efficiency, and, especially, the dura- 

 tion of the trial, he possesses no measure and can make no estimate. 



