1880.] 425 [Benton. 
siliceous rock, since all the different kinds of rocks which occur in it 
to any considerable extent contain usually at least sixty per cent. 
(.60) of silica. The principal constituents of this rock, however, 
contain still more silica. The felsitic and granitic or gneissic 
pebbles are at least seventy per cent. (.70) silica, while the quartzite 
which enters so largely into the composition of this rock is nearly 
pure silica. 
Thus it is safe to say that the conglomerate as a whole contains at 
least seventy per cent. (.70) of silica, while basalt on the other 
hand, contains only about forty-nine per cent. (.49) of silica, the 
highest analyses giving not over fifty-five per cent. (.55). Hence 
the change from one rock to the other would involve the removal of 
at least three-tenths (,3,) of the silica of the conglomerate. A 
further loss to the conglomerate of several per cent. of potash, and a 
gain in lime, soda, magnesia and iron would be among the other 
chemical changes necessary in order to have the proportion of the 
chemical constituents of the conglomerate made the same as that of 
basalt. So far as we know, however, the fusing of rocks does not 
materially affect the proportion of the chemical ingredients of the 
rock as a whole, and cannot change a highly siliceous rock to a highly 
basic one. : 
It seems probable then that the so-called pebbly structure in the 
amygdaloid cannot properly be regarded as evidence that any part of 
the amyedaloidal rock was derived from the pudding-stone. 
Most of the bodies which have been called pebbles are included 
fragments of the same kind of rock as that which includes them. | 
This rock shows a tendency towards a concretionary structure — a 
secondary structure not yet explained, but known to occur in some 
other igneous rocks. In parts of this melaphyre, the spheroidal 
agoregations of amygdules already mentioned are to be referred to 
that structure. 
Those bodies in the melaphyre which are truly included fracments, 
are probably the result of the breaking up of a pre-existing rock by a 
second intrusion of the same kind of igneous matter along the same 
path. 
The occurrence of many of the spheroidal ageregations as included 
fragments seems to point to the fact that a spheroidal concretionary 
~ structure was already developed before the rock was broken up by 
the second flow; it thus had a tendency to break into spheroidal 
forms, and on consolidation of the whole, these bodies preserved ther 
