STATE GEOLOGIST. 99 
surface percolation to be admitted to the filter beds and to be filtered with 
the regular supply. Concrete manholes opening to the surface are con- 
structed at regular intervals to allow light and ventilation during clean- 
ing and reconstruction of the sand beds. [Figure 41 shows a section 
through the filter bed in the Roxborough plant, Philadelphia, which is 
similar to the Belmont plant. 
Fig. 42.—Floor of Filtration Chambers in Process of Construction, Philadelphia, Pa. 
Figures 42, 43 and 44 illustrate the progressive steps in construction 
of the filter plant. 
DAMS. 
Concrete is especially useful in the construction of dams. Usually 
these must, of necessity, be located at points not easily accessible for se- 
curing cut stone and therefore for economy, concrete presents the best 
available material. Usually, however, stone is in abundance in some form 
which can be crushed into concrete size. It is now known: that crushed 
stone screenings are as good as sand for a fine material to fill voids and 
make mortar, so that all the material requiring transportation to the site 
is cement. Solid monolithic structures can thus be erected with great 
economy. ) 
