20 THE ATRIUM VEST. 
used occasionally, however, in certain of the walls on the opposite side.* 
The upper portion of these walls is, however, in all probability, a restoration 
of the succeeding period. With the rise of the new city a distinct change 
takes place in the type of brick-facing used for concrete structures. The 
courses of brick, which are 3.75 to 4.25 centimeters wide, show much less 
variation in the individual walls. The layers of mortar also, which are 1.25 
to 2 centimeters thick, become more regular. The change, or rather develop- 
ment, is especially noticeable in the character of the bricks themselves. 
The use of broken tiles for bricks, which was the most marked characteristic 
of the preceding period, was abandoned,’ and the bricks assumed the more 
regular triangular shape. The new bricks are of a much coarser quality 
than the tile-bricks and are much looser in texture, though they are well 
burned. In color they vary from yellow to yellow-red. ‘The concrete, also, 
is of a coarser composition, though otherwise of a good quality. The filling 
is in large part of tufa and travertine with but a small proportion of broken 
brick and marble. Both mortar and concrete are marked by their unusual 
gray tone, which arises from the almost entire absence in their composition 
of red pozzolana.’ 
1 The walls in which the bonding-courses appear In the description of the modes of construction 
most frequently are those east of the tabli- and materials used in the different periods, 
num. Bonding-courses appear sporadically all minor details have been omitted except 
in this period, but their use is not common > such as are necessary for the differentiation 
until the time of the Flavians. between the various periods. The whole 
2 At several later periods they are again used in subject will be treated fully by the writer 
considerable quantities. at a later time. 
