102 BULLETIN OF THE 
The beds above the sheet, as well as below, may be altered by heat. 
The alteration is commonly seen in change of color, induration, pro- 
duction of new minerals, or the development of a local prismatic habit. 
Strongly contrasted with all these are the features characteristic of 
extrusions :— 
An extrusive sheet lies conformably on the surface over which it was 
poured. 
The lower and upper portions are strongly unlike. 
The upper surface sometimes manifests a ropy flow structure, and 
sometimes consists of a mass of clinkers. 
Vesicular or amygdaloidal texture is very common, especially near 
the upper surface, and sometimes within the mass. 
A composite structure, as of two or more flows, is not uncommon. 
Vesicles are often drawn out in a common direction, parallel to the 
adjacent surface, and indicative of motion; but greatly elongated 
“spike” amygdules stand at right angles to the neighboring surfaces, 
These amygdules are commonly characterized by a definite boundary, 
and by a tendency to an arrangement of the adjacent feldspar crystals 
parallel to their walls, and are therefore regarded as the product of 
expanding gases. Pseud-amygdaloidal cavities are also common. 
There is a marked tendency to the development of a porphyritic 
structure throughout the whole mass. 
The overlying beds show no evidence of alteration by heat. 
The overlying sediments are arranged conformably with the upper 
surface of the sheet ; open vesicles and the spaces between clinkers are 
more or less completely filled with sediments, deposited conformably 
with the surface on which they rest. 
A stratified mixture of clastic materials and trap fragments, the latter 
more or less water-worn, overlies the sheet. 
Extrusive sheets may be associated with ash beds and volcanic bombs, 
and with beds of volcanic conglomerate, more or less water-worn, in a 
horizon nearly continuous with the lava sheet. 
It may be added, that the effects of heat and of mechanical disturb- 
ance in the underlying beds are features common to sheets of either 
intrusive or extrusive origin; and that absence of induration and ap- 
parently complete conformability with adjacent beds cannot be taken as 
proving extrusion. 
Induration is one of the most commonly quoted effects of the action 
