AGE OF PfROCRYSTALMNE ROCKS. 
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38. Age of ‘pyrocrystalline rocks. The consolidation of 
the earth’s crust resulted in the production of the pyrocrystal¬ 
line rocks. If any part of the cooled pellicle thus formed 
remains, that would be the oldest rock. A pellicle must have 
been ultimately formed, and which still maintains its existence 
as a constituent part of it. From the manner in which the 
surface cools, the consolidated masses which successively form 
must lie in contact with the inferior surface of the first-formed 
pellicle. The thickness of the crust increases by additions 
below. This mode of consolidation differs materially from the 
increase of the crust by additions to the hydroplastic rocks, as 
these increase by new overlying deposits—a mode by which 
the newest or latest formed rocks are superior; while in the 
former the newer are beneath and the older above. When, 
however, the crust contracts fissures are produced, through 
which the still fluid matter finds its way to the surface, and 
may overflow the consolidated surface. The age of two rocks 
thus related is determined by very obvious facts. The rock 
intersected by fissures and filled with melted matter must he 
the oldest, and the intersecting mass the newest. Three, and 
even more masses may be thus related to each other. This 
mode of formation, as well as the indications of age, belong 
exclusively to this class of rocks, and in a series of adjacent 
beds we are to look for these peculiar relations, when it is 
desirable to determine which is the oldest and which the newest. 
The rule has a general application, as it is obvious that all 
intersecting masses of rock must be more recent than the inter¬ 
sected, whether the latter belongs to the oldest or newest 
