698 C. R. VAN HISE 
to get definite notions of the common kinds of rocks. Having 
this knowledge, he knows without explanation the meaning 
which is to be attached to a name compounded of two of them. 
He knows without a moment’s reflection the meaning of a 
mineralogical qualifier prefixed to a noun. He knows that a 
geographical qualifier means the particular variety of rock which 
occurs in a certain district. Thus it will be possible for a student 
to gain a comprehensive knowledge of the chief kinds of rocks 
at a very early stage of work, and to supplement his general 
knowledge by details as his work advances. 
5. A further advantage of the foregoing plan will be that 
many of the earliest rock names proposed will be retained, 
although some may be abandoned. The reason why so many 
of the old names will be retained under the plan proposed is that 
many of the abundant rocks were first found; for, as already 
noted, while the petrographers supposed they were following the 
plan of naming rocks on the basis of types, they were largely 
following the plan of naming rocks on the basis of abundance. 
6. Still another of the manifest advantages of the plan is that 
it throws the emphasis on the proper thing; it calls attention to 
the common, not to the exceptional, rocks. Many petrographers, 
in discussing the rocks of an area, describe with the utmost 
particularity the minute characteristics which are peculiar to the 
exceptional rocks of the district or region, and say comparatively 
little of the common rocks which compose the great mass of the 
rocks of the area. Of course, there is some justification for this, 
in that the characters of the common forms are known. But all 
papers describing the geology of a region should be so framed 
as to give a clear conception of the common kinds of rocks 
which are preponderant, and indicate that the exceptional things 
described in such detail are only so described because of their 
peculiarities. To the reader in some way should be conveyed 
the idea of the relative abundance of the rocks. The error of 
emphasizing the exceptional and overlooking the common phe- 
nomena has been one of the most pernicious mistakes which runs 
throughout geological papers and text-books. For instance, 
