50 VOLCANOES 



It must not be inferred that the joints of all rocks are due to 

 shrinkage on cooling. It will be shown in a subsequent chapter 

 that such is very far from being the case. 



Not all the lava produced in and around a volcanic vent can 

 reach the surface. Some of it may be forced horizontally be- 

 tween the beds of the surrounding rocks, thus forming intrusive 

 sheets, which, when exposed in section, may be readily distin- 

 guished from surface flows by the fact that they have consolidated 

 under pressure, and hence have no slag or scoriae associated 

 with them. Other portions of the lava will fill up vertical fissures 

 in the volcanic cone or in the underlying rocks, and, solidifying 

 in these fissures, form dikes. Such a fissure, twelve miles in 

 length' and filled with molten lava, was observed by Sir Charles 

 Lyell in the neighbourhood of yEtna. In the great eruption of 

 Skaptar Jokul (Iceland) in 1783 lava was poured out at several 

 points along a line two hundred miles long, and doubtless this 

 was a great lava-filled fissure which consolidated into a dike. 



We thus see that the molten masses may not all well up 

 through the crater of a volcano, but will seek egress along the 

 line of least resistance, wherever that happens to be, breaching 

 the walls of the volcanic cone, rising up through vertical fissures, 

 or forcing their way as intrusive sheets between the beds of pre- 

 existing rocks. In these various situations the different rates of 

 cooling produce many varieties of rocks, though the original 

 molten mass may have been nearly or quite identical in all of 

 them. 



Lavas which flow into the sea from a terrestrial vent, or are 

 poured out from a submarine one show, as a rule, but little 

 difference from those which solidified on land, because the rapid 

 formation of a cindery crust will protect the hot lava from contact 

 with the water. Sometimes, however, the sudden chill will cause 

 the lava to disintegrate into a mass like black sand. 



(2) Fragmental Products. — This division includes all the mate- 

 rials which are ejected from the volcano in a solid state. These 

 are of all sizes and shapes, from huge blocks weighing many tons, 

 down to the most impalpable dust, which the wind will carry for 



