Dikes are commonly more fine of grain on the sides than in the center, and may have a glassy and crackled surface where they meet the inclosing rock. Can you account for this on any principle which you have learned?
Fig. 240. A Dissected Volcanic Cone
N, volcanic neck; l, l, lava-topped table mountains; t, t, beds of tuff;
d, d, dikes; dotted lines indicate the initial profile
Volcanic necks. The pipe of a volcano rises from far below the base of the cone,—from the deep reservoir from which its eruptions are supplied. When the volcano has become extinct this great tube remains filled with hardened lava. It forms a cylindrical core of solid rock, except for some distance below the ancient crater, where it may contain a mass of fragments which had fallen back into the chimney after being hurled into the air.
Fig. 241. Mount Johnson, a Volcanic Neck near Montreal
As the mountain is worn down, this central column known as the volcanic neck is left standing as a conical hill ([Fig. 240]). Even when every other trace of the volcano has been swept away, erosion will not have passed below this great stalk on which the volcano was borne as a fiery flower whose site it remains to mark. In volcanic regions of deep denudation volcanic necks rise solitary and abrupt from the surrounding country as dome-shaped hills. They are marked features in the landscape in parts of Scotland and in the St. Lawrence valley about Montreal ([Fig. 241]).