MANSFIELD: ROXBURY CONGLOMERATE. 153 
lated to include each feature that may be expected to appear. The 
definite statement of the absence of any such feature is often as impor- 
tant as the mention of the presence of another, since it indicates that 
its importance was recognized but that search for it was fruitless. Such 
a plan serves to stimulate observation and to direct attention to char- 
acteristics perhaps otherwise overlooked. 
In the following descriptions of the various rocks the writer has 
attempted to use the plan set forth (page 138). Since, however, the 
section entitled ‘Relations to Subjacent Rocks” applies to the larger 
relations of conformity and unconformity and cannot well be illus- 
trated in hand specimens, that topie will be replaced by “ Relations 
to Melaphyr.” 
Descriptive Terms. The descriptive terms now in use for the 
shapes of pebbles need more exact definition. In the present discus- 
sion the four following terms will be employed to express the compar- 
ative degree of rotundity attained by the pebbles of a conglomerate. 
Angular. The fragments present sharp angles or edges with little 
or no evidence of attrition (Plate 1, A). 
Subangular. 'The pebbles vary from almost angular fragments to 
smooth stones that have no sharp angles or edges but still 
retain relatively flat or irregular sides (Plate 1, B). 
Rounded. 'The pebbles begin to lose their flattened and somewhat 
irregular shape and to approach the form of the ellipsoid or 
spheroid (Plate 1, C). 
Well rounded. 'This term is reserved for pebbles that have acquired 
ellipsoidal or spheroidal forms. No specimen is at hand which 
illustrates this type well. Some of the pebbles in C and D 
(Plate 1) may be said to be well rounded. 
It is obvious that no sharp line of division can be drawn in the use 
of such terms and that the personal equation must therefore enter to 
some extent into all descriptions where they are employed. 
Tue ROXBURY SERIES.— Arkose. Arkose occurs in several parts 
of the Boston Basin. In Medford the conglomerate is described 
by La Forge as passing into arkose and granite (La Forge, p. 75, 
89). The arkose is somewhat kaolinized but consists of irregular 
grains of quartz and feldspar. Crosby speaks of arkose as occur- 
ring along the southern boundary of the Carboniferous series near 
the granite in Quincy, (n, p. 436). Outerops occur on Gun Hill 
Road and on Randolph Avenue (ibid., p. 438). This rock has not 
been seen by the writer but it is described by Crosby as quite 
