-1879.] 1383 [Shaler. 
afterwards covered by a later series of deposits; it seems more likely, 
however, that the ejection of the amygdaloid did not take place until 
the rock had been upheaved at a high angle, so that the line of easiest 
escape for the molten rock was along the planes of bedding — that is, 
it was easier for it to rupture its way out in this line than across the 
close textured bedding planes. 
Although there are thousands of dykes exposed in Eastern Massa- 
chusetts, there are very few that show any distinct traces of air 
vesicles in their structure. The mass in Brighton and one in Hing- 
ham being the only cases known to me where the vesicular charac- 
ter is distinctly indicated; both these masses seem to me pretty 
clearly eruptive in their origin, though by some excellent observers 
they are regarded as stratified rocks altered in place. 
It seems to me that this hypothesis of the passage of sedimentary 
beds into lavas, to which we are led by the inspection of the phe- 
nomena of these amygdaloids, is connected with all we know of the 
action of volcanoes in eruptions. Such eruptions consisting essentially 
in the escape of the gases of water and other volatizable substances 
which have been buried in sedimentary deposits, the generation of the 
gases must at first give rise to blebs such as we find in these 
amygdaloids which work their way through the mass to the point 
of escape. 
The absence of similar vesicles in the other more common dykes 
in this neighborhood is perhaps to be explained by the assumption 
which is verified by their other phenomena, that the ordinary dykes 
represent movements of lavas which have taken place in channels 
which did not communicate freely with the surface, and were under 
such great pressure that their volatilizable substances could not pass 
into the state of vapor. 
I should say that it seems to me likely that under the term amys- 
daloids we include rocks having a very diverse origin; those where the 
almond-like segregations arise from the infiltration of gas cavities, 
and others where the cavities are formed by a process of decay; some 
at least of the latter class are probably true stratified rocks which 
have never been melted. 
