GEOLOGY. 281 



consumed in the formation of the nitrate of lime must have its propor- 

 tion of free oxygen disengaged, thus enriching this subterraneous atmos- 

 phere with a larger portion of the exhilarating principle. The temper- 

 ature of the cave is uniformly 59, summer and winter, and this is 

 probably very near to the annual mean of the external air. The ex- 

 pansion which accompanies an elevation of temperature in the outer 

 air is immediately felt by the denser air of the cave, and it flows out in 

 obedience to the law of motion in fluids, and the outward current con- 

 tinues without interruption as long as the outer air is possessed of a 

 higher temperature than the cave. 



The phenomena of life within the cave are comparatively few but 

 interesting. There are several insects, the largest of which is a sort 

 of cricket, with enormously long antennae. There are several species 

 of coleoptera, mostly burrowing in the nitre earth. There are some 

 small species of water insects, supposed to be crustaceous. Of fish, 

 there are two species, one of which, as is well known, is entirely eye- 

 less ; the other has external eyes, but is quite blind. The only mam- 

 mal, except the bats, is a rat, which is very abundant. Prof. Silliinan 

 is of opinion that the excavations of the Mammoth Cave have been 

 formed by water, and by no other cause. 



NATURAL BRIDGE IN ALABAMA. 



Ix the recent geological survey of Alabama, by Prof. Tuomey, a nat- 

 ural bridge was examined, which is described as rivalling the celebrated 

 one in Virginia. The location is in Walker county, about a mile from 

 the road. " It occurs," says Prof. Tuomey, " in that geological deposit 

 termed the millstone grit the lowest one in the coal measures 

 the only rock which, in Alabama, exhibits the truly wild and romantic 

 grandeur of nature. Before reaching it, our imaginations had been 

 considerably elevated by the descriptions given by our guide ; but, not- 

 withstanding, when the reality broke upon us in its full magnificence, we 

 found that our expectations fell very far short of the truth. This grand 

 structure of the Great Architect spans about one hundred and twenty 

 feet, while its height is about seventy. A smaller bridge connects it 

 with the bluff beyond. The symmetry of the main arch will make it 

 almost indestructible, though, of course, its regularity has only been 

 produced by the undermining and breaking down of the rock which, 

 at some by-gone time, existed below it. The cleavage marks of the 

 massive sandstone, of which it is formed, causes it, even in the more 

 minute construction, to resemble an artificial bridge, as these lines 

 make it appear as if built with regularly worked blocks. Beneath it 

 are many pieces of broken and partially water-worn rock materials, 

 as it were, left by the builders ; and these, together with the mighty 

 escarpments round about, would impart a most grand aspect, even if 

 that were not produced by the bridge itself." 



TUSCARORA SOUR SPRING. 



Tms spring is situated in the Indian Reserve, about nine miles 

 south of Brantford, and three miles south of the bank of the Grand 



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