MUSEUM OF COMPARAXn^E ZOOLOGY. 101 



searched the region carefully for all the outcrops and openings that 

 might give opportunity of testing these deductive possibilities, and we 

 now present the result of this search. 



During the progress of our field and laboratory studies, the latter 

 having been carried on by the junior author, we have looked for the 

 results of similar studies in other regions. It appears from this that 

 the question as to the intrusive or extrusive origin of lava sheets is 

 seldom discussed in detail ; as a rule, it has been settled by the citation 

 of a few facts, without going through the greater labor of making com- 

 plete diagnoses. We cannot therefore always determine whether all the 

 criteria of intrusion or extrusion are present in the examples referred 

 to. Opportunity for observation is often limited ; search for outcrops 

 is frequently hasty ; but the criteria that are cited are as a rule dis- 

 tinctive. Putting all these together, we find that the facts indicative 

 of an intrusion are as follows : — 



An intrusive sheet is not confined to a single horizon, but may break, 

 across the adjacent strata. 



The lower and upper portions of an intrusion are nearly identical. 

 Offshoots may traverse the superincumbent beds for some distance from 

 the main sheet. 



The texture of the mass is, with small exception, dense throughout, 

 being uniformly and coarsely holocrystalline in the middle, but becom- 

 ing very close-grained and glassy close to the upper and lower surfaces, 

 with the development of marked porphyritic structure and of minerals 

 not observable in the middle, and non-polarizing action immediately at 

 the contact. 



A cellular or amygdaloidal texture is rarely developed, and when 

 occurring seems to be confined to the upper portion of the sheet. The 

 microscope generally does not discover a definite boundary or a tangen- 

 tial arrangement of feldspar crystals around tlie walls of these pseud- 

 amygdules, and their cavities are therefore ascribed to replacement. 



The porphyritic crystals of the upper surface are arranged tangen- 

 tially to the inequalities of the enclosing rock, showing the former to be 

 secondary to the latter. 



Enclosed fragments of the country rock may be found near the upper, 

 as well as near the lower, surface of the sheet. 



The overlying rocks, as well as the underlying, are fractured and dis- 

 turbed, and friction breccias are sometimes formed along the contact 

 surfaces, the fragments from the intruded and the enclosing rocks being 

 mutually and mechanically commingled. 



