786 A TREATISE ON METAMORPHISM. 



The foregoing' classification is made upon the basis of giving family 

 names to the abundant kinds of the sedimentary rocks. If names were 

 provided for kinds which are not abundant, a multitude of names would 

 be necessary. The intermediate varieties may be provided for by com- 

 pounding the various names within either the fragmental or the nonfrag- 

 mental class. If the materials of each of the two classes be about equally 

 abundant, the names ma}- be hyphenated; if one is subordinate, this may 

 be used in the adjective form. The following illustrates the usage: If a 

 rock is about halfway between shale and limestone, it may be called a 

 shale-limestone or limestone-shale. If the fragmental material be subordi- 

 nate, it may be called shaly limestone. If the nonfragmental material be 

 subordinate, it may be called a calcareous shale. In a similar manner, if 

 a rock be intermediate between the carbonate and siliceous rocks, one may 

 say limestone-chert, chert-limestone, or calcareous chert, cherty limestone. 

 If it be desired to indicate si ill more closely the character of the rock, 

 geographical names may be prefixed. 



It thus appears that the classification of the sedimentary rocks is 

 comparatively simple, since, as explained on pages 5o5-560, the sorting 

 of the sedimentary material, both mechanically and chemically, is along 

 definite lines, and consequently large masses of material are produced 

 which have comparatively narrow ranges in composition, and these abun- 

 dant rocks are selected for the family names. However, the fact must 

 not be forgotten that there are all gradations between all varieties of 

 the sedimentary rocks. The law that gradation is the rule in nature is 

 no better illustrated than by the sedimentary rocks. In biology, while 

 gradations have once existed between various forms, in many cases the 

 intermediate forms have been destroyed. In the early days of petrog- 

 raphy it was not supposed that between the various igneous rocks gradation 

 existed. But gradation varieties are known between the more important 

 facies, although gradational varieties between all kinds have not yet been 

 discovered. But the fact that each kind of sedimentary rock grades into 

 the related kinds has been recognized since the rise of geology, and thus 

 there has been no such confusion and multiplication of names for the 

 sedimentary l-ocks as for the igneous rocks. While gradations between 

 the different varieties of the sedimentary rocks exist, it is believed that 

 under the families above named probably 90 per cent or more of the 



