

198 STRUCTURAL AND FIELD GEOLOGY 
fragmental materials are extremely coarse—an aggregate of 
angular and subangular blocks and smaller stones in a matrix 
of finely comminuted débris, which may be meagre or relatively 
abundant. All the frag- 
ments may consist of 
crystalline igneous rock 
of one or more kinds, or 
these may be commingled 
with the débris of sedi- 
FIG. 71.—NECK OCCUPIED BY AGGLO- mentary rocks—the rela- 
ee AND CRYSTALLINE IGNEOUS {ive proportion of igneous 
i and sedimentary mate- 
rials varying indefinitely. Sometimes the contents of a neck 
consist of derivative rocks only, as sandstone, shale, limestone, 
ironstone, coal, etc. In necks composed mostly or exclusively 
of igneous materials, large broken crystals of various volcanic 
minerals sometimes occur, as hornblende, augite, biotite, sani- 
dine, pyrope, etc. Still more remarkable is the appearance 
in some tuff-necks of abundant small and larger fragments 
of coniferous wood. Although the fragmental materials 
are usually somewhat coarse, yet not infrequently these are 
associated in the same neck with areas of much finer grained 
tuff; while in some cases the whole neck consists of fine tuff, 
which now and again has been so altered as to assume a 
crystalline or subcrystalline aspect. 
The agglomerate and tuff often exhibit more or less 
distinct traces of a centroclinal dip—the materials being rudely 
bedded around the marginal area and inclined inwards towards 
the centre, where all trace of bedding is usually lost, although 
occasionally the coarse material appears roughly arranged in 
nearly vertical lines. Not infrequently it is about the centre 
of a neck that the larger blocks and stones are most abundant ; 
but in many necks no such aggregation can be traced. 
Massive dykes and branching veins of basalt or other 
crystalline igneous rock often pierce and ramify through the 
agglomerate and tuff. These may be confined to the neck 
itself, or pass outwards into the contiguous strata. The 
massive rock which often completely fills a neck is usually 
traversed by well-marked horizontal jointing, but in the case 
of large necks these joints are often confined to the marginal 



