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 mixt\xre 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 nearlv continuous with the lava sheet. 



It may be added, that the effects of heat and of mechanical disturb- 

 ance in the imderlying 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 



