MUSEUM OF COMPARATIVE 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 
eriteria 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 the 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. 
