STRUCTURE OF ERUPTIVE ROCKS Lon 
of two or more closely adjacent necks having coalesced. Not 
infrequently, fissures, filled with agglomerate or tuff, pass 
outwards from a neck into the adjacent rocks. More remark- 
able than these, however, are certain vertical fissures of 
eruption which occur independently, or seem, at least, to have 
no connection with necks or pipes. At the surface, these 
appear in groundplan as long, lenticular ribbons or belts, or 
they may expand and contract irregularly. They are filled 
with fragmental materials, and thus might be tersely described 
as agglomerate-dykes. Fissures of eruption of this kind are 
not common, and seem to be confined to regions where 
volcanic rocks are well developed. Isolated examples occur 
in the Sidlaw Hills and in South Ayrshire, and they are 
met with likewise in the Cheviot Hills. Necks often appear 
upon a line of fault or dislocation, but in many cases no such 
connection can be traced. Although they now and again 
occur singly, they more usually cluster in groups within a 
limited area. They vary much in size—some measuring only 
a few yards across, while others may be several hundred yards 
in diameter; exceptionally, they may reach or even exceed a 
mile in width. They usually form more or less abrupt knolls 
or isolated hills, which vary in shape according to the nature 
of the materials of which they are composed. Many are 
more or less conical; others are somewhat steep and not 
infrequently craggy ; while yet others are smooth and rounded. 
The rock occupying a neck may be crystalline, as basalt, 
andesite, phonolite, quartz-porphyry, felsite, etc. (see Fig. 69) ; 
) 



FIG. 69.—NECK OCCUPIED BY FIG, 70.—NECK OCCUPIED BY 
CRYSTALLINE IGNEOUS ROCK. AGGLOMERATE. 
or it may consist of fragmental materials, as agglomerate or 
tuff (see Fig. 70), or both fragmental and massive crystalline 
igneous rocks may be present (see Fig. 71). Frequently the 
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