GRADE AND CLASS TERMS FOR CLASTIC SEDIMENTS 387 



of the writer and of many other geologists that nearly all clastic 

 materials of this grade consist largely of the hydrous aluminum 

 silicates which make up the clay of the chemist and also that the 

 material is always more or less plastic. There is, therefore, in his 

 opinion a common ground for the geologist and chemist without 

 an insistence on the use of the term clay for the pure chemical 

 compounds kaolin or other minerals of this group. 



ROCK TERMS 



Conglomerate. — There are certain terms which are used with 

 greater uniformity and less abuse than others. One of these is 

 conglomerate. This term is very widely applied to rocks which 

 are the consolidated equivalents of gravels. It is applied just as 

 is the term gravel to rocks which vary widely in the sizes of their 

 constituent particles. It seems desirable, therefore, to apply to it 

 modifying adjectives as has been done with gravel, making the terms 

 bowlder-conglomerate, cobble-conglomerate, pebble-conglomerate, 

 and granule-conglomerate. The term granule-conglomerate is 

 preferred to the term grit because grit has been used in England 

 for both coarse- and fine-grained and angular-grained sandstones. 

 The use of the term grit in the present sense seems therefore 

 inadvisable. 



Sandstone. — This term is generally used by geologists and no 

 great change in its usage is here proposed. It is qualified by the 

 adjectives very coarse, coarse, medium, fine, and very fine, and 

 used for the consolidated equivalents of the various grades of sand. 

 The term grit is not used for the coarser grade for the reasons 

 stated in considering the term granule-conglomerate. 



Siltstone. — After a consideration of a number of terms, siltstone 

 was adopted by the writer as most acceptable for the consolidated 

 equivalent of a silt. Shale, as proposed by some geologists, was 

 considered objectionable because, in the usage of a majority of 

 geologists at present, as well as etymologically, it is a structural 

 term referring to the shelly structure of the rock rather than to 

 the size of its grains. 



Claystone. — The term argillite has already been used in another 

 sense and the term shale is objectionable for the reasons given 



