Shaler.] 130 [January 15, 
clear that these spots were originally cavities, into which their present 
contents have been carried by the action of water. The magnifying 
glass, and at times the naked eye, show the successive coats of ma- 
terial which were deposited as the holes were filled up. Often as the 
filling went on, there came a change in the materials borne by the in- 
filtering waters, so that the inner spaces of the cavities contain 
other substances than the outer circles. In localities near the 
pumping house quarry, especially in those on Allston Street near 
the carriage factory, this amygdaloidal rock shows the cavities 
partly filled with a reddish jaspery substance. The upper level 
of this filling is a plane; above it to the top of the cavity subsequent 
infiltation has brought in other substances. At other points near 
the small cemetery on Washington Street, about four hundred 
metres south from the pumping house quarry, and at the quarry on 
North Beacon Street, thirteen hundred metres nearer Boston, we 
have yet another set of facts which serve to show more clearly the 
history of this peculiar rock. We see in these localities a mass of 
conglomerate essentially the same as the Roxbury pudding stone, 
only the pebbles and the cement have been greatly affected by heat 
so that the whole is more fused together than in the ordinary forms 
of that conglomerate — looking closely we see that the matrix of the 
pebbles and to acertain extent the outer parts of the pebbles them- 
selves are filled with cavities in which similar amygdules have been 
formed. With care and with favorable conditions of the quarries, 
the observer may trace the stages of this transition, from the faintest 
beginning of this structure in rocks which are distinctly conglomer- 
ates, into rocks where the blebs have been so completely developed 
that every trace of the original pebbly structure is now lost, and the 
mass converted into the amygdaloidal trap. ‘The trained eye also 
recognizes that as the blebs increased in the pebbly mass it became 
endowed with a certain plasticity and was squeezed out of its orig- 
inal position. The direction in which it moved is also traceable by 
the elongated forms of the blebs or of the amygdules which have been 
deposited in them. 
By carefully following these lines of evidence the student will be 
forced to conclude that the history of these amygdaloids is about as 
follows: In the first place the pebble beds and slates of the 
Roxbury conglomerate series were laid down to the thickness of 
several thousands of feet of strata, on them came many thousand feet 
of other beds which once covered this region but have since been 
