RECAPITULATION. 33 
Such viscous materials as these acid rocks must have been when molten 
would solidify into more or less bulky, erect masses of relatively small 
area; and their degradation into geologically contemporaneous conglomer- 
ates seems to have been chiefly rendered possible by this mode of occur- 
rence. §trewn as they were around the rim of a basin whose middle por- 
tion was steadily depressing during the growth of the series and whose 
edges were at the same time rising, each of these bulky masses was able 
to supply the materials for a number of different horizons of sandstone and 
conglomerate. 
3LS8 
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