The masonry of the early =chancel= is, however, entirely different, being composed of Roman bricks laid evenly upon one another with narrow joints, averaging four bricks to a foot. In many instances the arrises of these bricks are sharp and true, showing no sign of having been taken from any other building; in other cases they are more fragmentary, but we can have no hesitation in saying that the walling of the early chancel is well-built, satisfactory to a professional eye. We have then these two distinct modes of building (1) Roman bricks laid evenly and closely upon one another, (2) stone-work with courses of Roman brick at various intervals. And we shall have to consider hereafter whether these are genuine Roman walls, or are merely composed of Roman materials used up for the second time, as at St. Albans and elsewhere. We learn from competent authorities that there were five or six kinds of Roman wall-building--(_a_) The _quadrangular_, with masses of square or oblong stones laid alternately lengthwise and cross-wise, not cemented by mortar, but bound together by leaden clamps, such as is found in the so-called wall of Romulus on the Palatine; (_b_) _polygonal_ masonry, where the stones are irregular, and with small stone splinters wedged into the joints where necessary; (_c_) _concrete_--rude, without ornamentation, which has at a distance the appearance of being panelled, since beams of timber are let in to strengthen it, or sometimes thin layers of brick to prevent settlement in the concrete from the shrinking of the lime when it cools and dries; (_d_) _opus reticulatum_, which consists of stone net-work of diamond-shaped blocks, as in the "Muro Torto" at Rome; (_e_) _opus lateritium_, the ordinary construction of bricks laid evenly upon one another (_f_) _mixture_--_i.e._ stones bonded together with courses of bricks, sometimes at regular, often at irregular, intervals. Mr Parker, in his "Archæology of Rome," referring to the _mixture_ (_i.e._ the style of the building used in the nave) which is so constant in Roman wall-work, in England and Northern Europe generally, says that in itself it is no evidence of date as to the period of Roman work, since other things must be taken into account: but that it is found in the circus of Maxentius, and many other places. It is usually attributed to the beginning of the fourth century, but it occurs also at Pompeii, in parts of the substructure of the walls of Aurelian, in tombs of the second century at Ostia, and in some of the foundations of Hadrian's villa near Tivoli.