1880.] 493 (Benton. 
The amygdules are also a subsequent product. In the great part 
of the rock they are not bounded by any well defined wall, and 
furthermore there is no arrangement of the feldspar crystals with 
reference to their boundaries. Cavities formed by gas bubbles would 
have a sharply defined wall, and the pressure of the gas causes the 
adjacent feldspar crystals to arrange themselves parallel to the 
boundaries of the cavities. 
It is probable that nearly all these amygdules in the rock are 
rather pseudo-amygdules, formed not by infiltration into pre-exist- 
ing cavities, but by a breaking up in spots of the original constit- 
uents, and the formation in siiu of these new products. 
A small proportion of them, however, show by their sharply 
defined walls, and by the arrangement of the surrounding feldspar, 
that they were formed in true gas cavities. 
Reference has already been made to the supposition that the 
Brighton amygdaloid was derived from the Roxbury pudding-stone 
or conglomerate. As the derivation of so basic a rock from one so 
silicious, is on the face of it improbable, and as the writer’s obser- 
vations lead to quite a different conclusion, it is necessary at this 
point to consider the question more fully. 
Prof. Shaler does not maintain that we can actually trace in the 
field a continuous gradation from the unaltered pudding-stone to the 
genuine melaphyre. Hence it is not necessary to enlarge upon the 
fact that the field observations demonstrate that no such continuous 
gradation is to be observed. His view of the matter can perhaps 
be best stated by further quotations from the paper already referred 
to. ‘“ We see in these localities (namely: exposure near small ceme- 
tery on Warren Street, and at the quarry corner of North Beacon 
St. and Harvard Ave.), 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 a certain extent 
the outer part of the pebbles themselves, are filled with cavities in 
which . . . amygdules have been formed.” “In the quarry on 
North Beacon Street there is evidence that some of the large boulders 
of the conglomerate have escaped the complete destruction arising 
from the expansion of the contained gases.”’ 
Thus the evidence for the transition rests upon the existence in the 
amyegdaloid*of the remains of pebbles of the pudding-stone, and upon 
