PHYSICAL CHANGES OF THE OUTER SHELL 81 



will surprise no one to learn that the mineral grains are there 

 crushed into minute fragments and packed closely together. 

 Firm quartzites may be mashed or granulated until all trace 

 of the original sand grains is lost; and in coarse gran- 

 ites scarcely a crystal may be left in its original size or 

 shape. 



But other influences besides pressure are at work. Many 

 things indicate that the earth is hot within. The deepest 

 mines are uncomfortably warm even in midwinter. Vol- 

 canoes and hot springs scattered widely over the surface tell 

 of much greater heat beneath. In mines and borings 

 the temperature rises on the average about 1 for every 

 60 to 90 feet of descent. If this rate holds good, the 

 rocks in the zone of flowage should be hotter than 350 C. 

 (= 662 F.). 



Water is another factor. Although it is true that the 

 circulation of water in this deep zone is greatly impeded by 

 the general lack of cracks and pores, yet water is every- 

 where present ; and, at such high temperatures, it may 

 be in the form of steam, notwithstanding the great pres- 

 sure. 1 



This superheated water or steam is a powerful solvent. 

 In it many minerals dissolve, and they may crystallize out 

 again in new forms, which are better adapted to the pressure. 

 The new minerals produced are usually heavier and denser 

 than those which were dissolved, for in a dense mineral the 

 same amount of material occupies less space, a change de- 

 manded by the overpowering pressure. 



As the new minerals crystallize, it is easier for them to grow 

 at right angles to the greatest pressure than directly against 

 it. Thus all the crystals generally come to be elongated in the 

 same direction, and the rock takes on a banded or streaky 



1 Water passes into steam at 100 C. (212 F.) under the ordinary pres- 

 sure of the air at sea level. Under twice that pressure it boils at 120 C. 

 (248 F.) ; and at ten times the air pressure, at 180 C. (356 F.). Above 

 356 C. (673 F.) steam cannot be forced into the liquid state by any pres- 

 sure that has ever been applied. This is called its critical temperature. 

 B. & B. GEOL. 6 



