2 SOILS. 



Such effects may often be strikingly observed on small sur- 

 faces of compound crystalline rocks, such as granite, exposed 

 on glaciers, where the daily changes of temperature are often 

 extreme, viz., from below the freezing point to as much as 

 130 degrees Fahrt (54.4 degrees C). In such cases one may 

 sometimes scoop off the disintegrated rock by the handful, 

 while yet the mineral surfaces are almost perfectly fresh. 



On a larger scale, the disruption and scaling off of huge 

 slabs of granite, and rocks of similar structure, may be observed 

 in southern California on the southwestern side of rock ex- 

 posures, where slabs from a few inches to ten and more feet 

 in length and eight or ten inches thick, have slid off, perhaps 

 still leaning' against the parent rock, which has been rounded 

 off by a succession of such events into the domelike form so 

 characteristic of granite mountains. Merrill J reports similar 

 exfoliations to occur especially on the peninsula of California, 

 on Stone Mountain in Georgia, and elsewhere. 



A striking exemplification of the effects of frequent and 

 rapid changes of temperature on rocks, and of humid and dry 

 climates as well, is seen in the case of the great monoliths of 

 Egypt, one of which now stands in the Central Park, Xew 

 York. In the quarries of Syene in Upper Egypt, where most 

 of these monoliths were obtained, the rough blocks that were in 

 progress of quarrying when the work was abandoned, quite 

 two thousand years ago, still show an almost perfectly fresh 

 surface; and the same is true of the finished obelisks in Lower 

 Egypt, where both the changes of temperature and the rain- 

 fall are somewhat greater. It is a matter of public note that 

 one of " Cleopatra's Needles " which was set up in Central 

 Park nearly thirty years ago, but originally erected at Helio- 

 polis on the Nile, is in great danger of destruction from the 

 influence of a totally different climate, in which both the 

 temperature changes and the rainfall are much more frequent 

 and severe than in Egypt. The large crystals of feldspar and 

 quartz which compose the (syenite) rock material have had 

 fine fissures formed between them by often-repeated expansion 

 and contraction ; which when filled with water and subsequently 



1 See Rocks, Rock-weathering, and Soils, page 246 ; also paper on Domes and 

 Dome Structure, by G. K. Gilbert, in Bulletins of the Geol. Society Am., Vol. 15,. 

 pp. 29-36. 



