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560 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox wen 
volcanic districts of Britain necks not infrequently are filled with 
some siliceous crystalline rock, such as a quartz-porphyry or felsite, 
even where the surrounding lavas are basic. The great vent of the 
Braid Hills near Edinburgh, belonging to the time of the Lower 
Old Red Sandstone, is filled with felsite tuff containing 70 per cent. 
of silica, where the lavas which flowed from it are basic porphyrites 
with not more than 50 per cent. of this acid. Azain,at Largo in Fife, 
strings of quartz-felsite occur in one of the necks, though all the sur- 
rounding lavas are basalts. Necks of agglomerate and fine tuff abound 
among the Carboniferous and Permian volcanic regions of Scotland, 
and are laid bare in so many admirable sections, that these regions 
may be regarded as typical for this kind of geological structure. 
The fragmentary materials in necks consist mainly of different 
lava-form rocks imbedded in a gravelly peperino-like matrix of more 
finely comminuted débris of the same rocks; but they also contain, 
sometimes in abundance, fragments of the strata through which the 
necks have been drilled. Occasionally, as in some of the Maare of the 
Hifel, these non-volcanic fragments constitute most of the débris 
(p. 243). When this is the case we may infer that after the first gaseous 
explosions, the activity of the vent ceased, without the rise of the lava 
column or its ejection in dust and fragments to the surface. So 
unchanged are many of the pieces of sandstone, shale, limestone, or 
other stratified rock in the necks, that they have evidently never 
been exposed to any high temperature. In some cases, however, 
considerable alteration is displayed. Dr. Heddle, from observations . 
in Fife, concluded that the altered blocks in the tuff there must 
have been exposed to a temperature of between 660° and 900° Fahr.? ~ 
Among the numerous vents of central Scotland pieces of fine 
stratified tuff not infrequently appear in the agelomerates. This 
fact, coupled with the not uncommon 
occurrence of a tumultuous, fractured, 
and highly-inclined bedding of the 
tuff with a dip towards the centre of 
the neck (Figs. 287, 288), appears to 
show that the pipes were partly filled 
up by the subsidence of the tuff con- 
solidated in beds within the crater 
and at the upper part of the funnel. 
Further indication of the probable 
F1.288.—PLan or Neck, on Snore, SUbaerial character of the tuff is 
ar Exum, Fire. ’ furnished by abundant pieces of en- 
T’, tuff; the arrows marking the inward closed coniferous wood, which may 
Dip; §, sandstones through which have belonged to trees or brushwood 
BB, basalt dykec. blown opens that grew upon the dry slopes of the 
cones ; for these fragments are seldom 
to be seen in the estuarine and marine strata, out of which the cones 
rose, 

' Trans, Roy, Soe. Edin, xxviXi. p, 487. 
