PHYSICAL CHANGES OF THE OUTER SHELL 83 



baked into a dense flinty rock in which the microscope shows 

 that an abundance of little crystals have formed. Since pres- 

 sure may be much less effective here than in the deep zone, 

 the rocks altered along volcanic contacts do not generally pos- 

 sess the cleavage and banded structure which are the results 

 of recrystallization under great compression. 



The metamorphic cycle. When the changes of the two 

 zones are put together, it is seen that they form part of a 

 nearly complete cycle of alterations. By way of illustration 

 let us take an igneous rock, such as granite. In the belt of 

 weathering it decays ; the complex minerals such as feldspar 

 and mica are changed into simpler chemical compounds, and 

 of these some are dissolved, while the remainder, with the 

 unchanged quartz, forms soil. When carried away and 

 assorted by water, this residue makes beds of sand and clay, 

 and at the same time some of the dissolved substances are 

 deposited as limy ooze. Thus far the process is destructive. 



Gradually buried by more sediments, the sand, clay, and 

 ooze come to lie in the belt of cementation and later in the 

 zone of flowage. In the former they are consolidated, through 

 the processes of cementing and crystallizing, into firm sand- 

 stone, shale, and limestone ; in the latter, the simple minerals 

 of which they are composed are mashed, recrystallized, and 

 combined in the form of more complex minerals which to- 

 gether form solid crystalline rocks, in some respects not 

 unlike the original granite. Here the change is constructive. 

 If by still deeper burial the rocks could be heated to the 

 melting point, they might actually be made over again into 

 igneous rocks and thus complete the cycle. Whether this 

 has occurred, however, is doubtful. 



QUESTIONS 



1. Figures 64 to 67 are maps on which the outcrops of the strata 

 are shown as bands. The dip and strike are indicated by the usual 

 sign. The beds are numbered in the order of their age, " 1 " being 

 the oldest. 



