COMPLETENESS OF CEMENTATION. 619 



to the surface since early in Tertiary time. The best illustration known to 

 me of a completely cemented great formation of Tertiary age is the San 

 Juan breccias of Colorado. This formation is 1,500 meters thick; it was 

 very porous, and yet every ancient opening not of microscopic size, from 

 great fissures to pores between the particles of ash, is completely filled. 



Of course, after cementation has been partially or wholly completed 

 orogenic movements may occur which produce a new set of openings, and 

 a vigorous circulation be set up in the new openings. This would give 

 new trunk channels for circulation and thus assist in the cementation of 

 partially closed old openings. 



In judging of the nearness to completion of the process of cementation 

 when the rocks were at a considerable distance below the surface, openings 

 which have been produced by later orogenic movements when the rocks 

 were nearing the surface must be ignored. Commonly such ojDenings are 

 not closed. 



It is explained on pages 595-597, 646-648 that the openings in the 

 rocks are to some extent closed by pressure, and locally are closed by 

 injection. So far as these processes take place the space left to be filled by 

 the process of cementation is decreased; but the residual actually observed 

 to have been filled by the process of cementation is enormous. 



In the cementation of the openings between the mineral particles the 

 new mineral material may be added in two different ways. It may attach 

 itself to the old grains of like mineral character, or it may be deposited as 

 independent interstitial material. If mineral particles be fractured, it may 

 heal them with the same or some other mineral. 



When interstitial mineral material attaches itself to an old mineral of 

 like character, the two being in optical continuity, the mineral is said to 

 have been enlarged. The principle explaining the enlargement of old min- 

 eral particles rather than the development of new smaller mineral particles 

 is essentially the same as that explained on pages 74-76, that large indivi- 

 duals grow at the expense of small ones. Where there are old nuclei which 

 can be used the solutions deposit material upon these; for if independent 

 particles begin to form, these under the principle above referred to would be 

 likely to be again dissolved and deposited upon the old larger particles. 

 The enlargement process is far more important for quartz than for any 

 other mineral, although enlargements of feldspar, hornblende, augite, biotite, 



