

STRUCTURE OF ERUPTIVE ROCKS 199 
area, the rock towards the centre being, as a rule, very 
irregularly jointed. 
The strata in immediate contact with a neck are often 
bent over suddenly, so as to dip abruptly against the old 
pipe of eruption—not infrequently, indeed, they are quite 
vertical, and sometimes much jumbled and broken, large 
blocks, slabs, and reefs having been detached or partially 
detached from the walls of the neck, so as to become 
enclosed wholly or partially in the tuff and agglomerate, 
while irregular veins of tuff pass outwards into the contiguous 
strata as if filling rents and fissures. Now and again, so 
great is the confusion that it is hard to follow the actual 
junction between the neck and the contiguous rocks. In 
such cases it seems as if the wall of the old funnel had 
collapsed and fallen in. The effect of heat upon the rocks 
abutting upon a neck are sometimes very notable—sand- 
stones for a few yards away being converted into quartzite, 
and shales baked into a kind of porcellanite. On the other 
hand, not infrequently no alteration of any kind can be seen, 
coal having sometimes been mined close up against a neck 
without showing any trace of having been subjected to the 
action of heat. In other cases, coal has been rendered quite 
useless for many yards away from a neck, changed in fact 
into a soft, sooty substance. The amount of alteration pro- 
Gmeed) bears no relation to<the size of a neck; for while 
much change may occur round a small one, little or no 
alteration may be visible round one of much larger 
dimensions. 
Explanation of Phenomena.—The necks described above obviously 
indicate the sites of former volcanoes. Many occur along lines of 
dislocation, just as is apparently the case with not a few volcanoes at 
the present time. On the other hand, a large number of necks seem to 
have no connection with any lines of weakness, and such pipes of 
eruption, therefore, must have been blown or blasted out by escaping 
vapours. Many necks probably indicate small puys—products ofa single 
eruption, from which only loose ejecta were emitted. From others, one 
or more flows of lava have taken place. When the tuff and agglomerate 
of a neck consist wholly or largely of igneous materials, it is obvious 
that molten matter must have risen in the throat of the old puy, although 
it may never have flowed out as a lava. It is quite possible, however, 
or even probable, that lava-streams may have proceeded from many 
necks, around which no remains of such flows now exist. Subsequent 


