beds have been indurated into porcellanite ; 
Parr VIL. Secr. i. § 4.] VOLCANIC NECKS. 561 
It is common to find among necks of tuff, numerous dykes 
and veins of lava .which, ascending through the tuff, are usually 
confined to it, though occasionally they penetrate the surround- 
ing strata. They are often beautifully 
columnar, the columns diverging from the 
sides of the dykes and being frequently 
curved. - 
Proofs of subsidence round the sides of 
vents may often be observed. Stratified =: 
rocks through which a voleanic funnel has 1S | 
been opened commonly dip into it all round, | 
and may even be seen on edge, as if they 
had been dragged down by the subsidence 
of the materials in the vent. Beautiful 
examples occur along the shores of the 
Firth of the Forth.t. The fact of subsidence 
beneath modern volcanic cones has been 
already referred to (pp. 232, 243). - 
Kffects on Contiguous Rocks.— 
The strata round a neck are usually some- 
what hardened. Sandstones have acquired 
sometimes a vitreous lustre; argillaceous 
B 
3 
=! 
— 
iy 
Uhh 
in 
—— 
hse 
idea 
LU Ti 
—— 
iti 
LOT 
SSS 

, Bedded tuffs, &c.; T, Tuff of the 
coal-seams have been burnt and rendered 
unworkable. The coal-workings in Fife and 
_ Ayrshire have revealed many interesting 
- examples of these changes, which may be 
partly due to the heat of the ascending 
column of molten rock or ejected fragments, 
partly to the rise of heated vapours, even 
for a long time subsequently to the volcanic 
explosions. Proofs of a metamorphism pro- 
bably due to the latter cause may sometimes 
be seen within the area of a neck. Where 
the altered materials are of a fragmentary 
character, the nature and amount of this 
_ change can be best estimated. What was 
originally a general matrix of volcanic 
dust has been converted into a crystal- 
line and even porphyritic mass, through 
which the dispersed blocks of the agglom- 
erate, though likewise intensely altered, 
are still recognizable. Such blocks as, 
from the nature of their substance, must 
have offered most resistance to change,—pieces of sandstone or 
quartz, for example,—stand out prominently in the altered mass, 
1 Trans. Roy. Soe. Edin. xxix. p. 469. For an excellent example from New Zealand 
see Q. J. Geol. Soc. 1860, p. 245. 9 
O 
Fic. 289.—Srction across THE Bryn or Burnrisuanp, Fire. 
great neck of the Binn; B, Basalt veins. 
1, Sandstones; 2, Limestone; 3, Shales, &c.; b b, Interstratified basalts; ¢ ¢ 

