1879.] 131 [Shaler . 
worn away. In yet later times the mountain building forces 
crumpled these rocks into lofty mountain chains which have since 
been worn down to their very roots. The deep burial of the con- 
glomerate series brought a great heat into them; they were raised to 
a temperature much exceeding that of boiling water, possibly to a 
temperature of one thousand degrees Fahrenheit or more. Every 
part of these rocks contained small cavities in which water was stored 
away; these microscopically small water spaces were especially 
abundant in the matrix which inclosed the pebbles, but they occurred 
also in the pebbles themselves though less abundantly. When the heat 
to which these rocks were subjected by their deep burial beneath 
strata of subsequent deposition had exercised its full effect upon this 
mass, we may conceive these drops of water were straining to burst 
their bonds and take the state of vapor. In this state the rocks 
softened by heat, and under the strain arising from the aggregate 
pressure of these infinitely numerous steam vesicles, it only needed 
that some fissure penetrating from the surface, after the manner of 
the faults which rive the rocks in every disturbed country, should 
give a possible line of escape for the compressed vapor, to make the 
beginning of a volcanic eruption. Every time such a fissure opened 
the least prospect of relief these confined gases would blow the 
softened rock up into the crevice, as the rock rose the vesicles 
would expand under the relief from pressure to make the bleb-like 
openings in which the amygdaloids have been deposited. If the 
fissure led to the surface we would have had a true volcanic eruption; 
if the crevice did not rupture all the overlying rocks we should have 
only a dyke ejection. Although the exposures here are few and 
small, a little careful looking will show the observer many evidences 
of the steps by which this gradual passage of sedimentary rocks into 
lavas is brought about; in the quarry on North Beacon Street, just 
east of the crossing of Harvard Avenue, there is evidence that some 
of the larger boulders of the conglomerate have escaped the complete 
destruction arising from the expansion of the contained gases. In 
the unquarried rocks near the small cemetery on Warren Street, be- 
tween Breck and Cambridge Streets, the change of the rock from 
the expansion of the gases appears to be just begun and the air blebs 
were only found in the matrix which holds the pebbles, not having 
extended to the pebbles themselves. At the quarry on Allston 
Street between North Beacon and Warren Streets, in the pit of which 
a small carriage factory has been built, there is a mass of this rock 
