Delta-Plain at Andover, Mass. — Mills. 163 
fair inference from the name ''sand-plain," for one to suppose 
that they are composed of assorted sands. In treating this 
specific subject, students are liable to have in mind only the 
restricted locality which they describe. While no student in 
New England would mistrust the significance of the term 
"sand-plain," nor those in Pennsylvania that of "gravel-plain," 
such terms are neither descriptive nor accurate for localities 
where flat stones are one of the most conspicuous features. 
If we use the word sand correctly as an absolute and not rela- 
tive term, the application is incongruous. Any specific desig- 
nation as to texture is too narrow and limited to hold good, 
except for a given locality. A finical distinction, however, 
is neither useful nor practicable. A term that is both descrip- 
tive and generically true would seem, on the other hand, to be 
advisable. The important and interesting fact is structure 
rather than texture. That they were originally sub-aqueous 
deltas, which, under changed conditions, now assume the form 
of elevated plains, is the fact worthy of attention and study. 
If such has been their cycle of evolution they are homologous 
topographic forms, regardless of texture. For such topo- 
graphic forms professor Davis long since proposed the name 
of delta-plain or delta-plateau, instead of sand-plain,* a des- 
ignation which has also been employed by many others, being 
both suggestive and descriptive. While such a designation has 
been advocated for purely topographic reasons, there would 
seem to be a further plea, as already suggested, on the ground 
of texture. Differences in the latter may be due to differences 
in structure and composition to be found in the bed rock over 
which the ice-cap rode, and from which it largely collected its 
debris ; as well as to the elevation, slope, and general relief of the 
region in which a given plain may be found. The abrasion and 
erosion of the hard, deeply bedded geneissic and granitic rocks 
of Xcw England, were a different task from the corrosion of 
much jointed, thinly bedded and soft shales, or limestones, of 
other regions. The residue collected and dispersed, largely in 
open valleys in New England, where the bulk of dposits is 
quartz grains, presents a different aspect from the wash plains 
and delta plains of central New York. In the latter case the 
valleys are narrow, high walled, with a moderately steep slope, 
* Proc. Boston Soc. Nat. Hist., vol. xxv, 1892. 
