HAWORTH. | Physical Properties of the Tertiary. 275 
beds by no means are confined to such lenticular masses, but fre- 
quently appear as thin masses of clay spread out between layers of 
sand or gravel. 
The largest proportion of clay in the aggregate is undoubtedly 
intimately mixed with the finer grains of sand and silt, so that the 
plasticity is partially destroyed, and the general properties greatly 
altered. It is doubtful in fact if many localities can be found where 
a section would not show that at least nine tenths of the vertical 
thickness is partially composed of clay. 
PLAINS MARLS. 
Professor Hay introduced the term “plains marl” to designate 
a large part of the Tertiary materials of the plains. Over a large 
proporticn of the whole Tertiary area the upper part is composed of 
a variety of soil similar in almost every respect to the glacial soils: 
of northern United States. The most abundant ingredient of such 
a soil is clay, the remainder of the prominent constituents being 
fine sand, a little organic matter, and some products of decompo- 
sition of the surface material producing soils. In general character 
this is surprisingly similar to the glacial loess so well known in 
many parts of the world. There is no essential difference between 
it and many forms of soil and clay found in the valleys, ravines, and 
streams of various sizes in other parts of the state, excepting that 
the proportion of sand may at times be greater here, and the felds- 
pathic material is found in greater quantity. This loess-like material 
differs in character from the mortar beds to a considerable extent. 
Professor Hay has divided the whole Tertiary into two general 
classes, the mortar beds and the plains marls. For a convenient 
term to designate the difference in physical properties of the differ- 
ent Tertiary materials no objection is apparent to the use of either 
name. It should not be assumed, however, that the plains marl has 
any more definite stratigraphic position than the mortar beds have. 
Many masses of clay and sand admixtures found 100 or 200 feet 
beneath the surface would soon become typical plains marl were 
they exposed to the surface long enough to be acted upon by the 
atmosphere, and to receive a slight admixture of organic matter. 
Plains marl by no means covers all the surface. A considerable 
