232 WHITMAN CROSS 
designation. Such characters testify that a magma of certain 
chemical composition consolidated at the place where a given 
rock is seen, under conditions producing the mineralogical and 
textural result observed. What some of these conditions were 
can be inferred, and something of the eruptive history may be 
read in the petrographic characters. But these features give 
but little more data bearing upon geological chronology than do 
those of a sandstone or conglomerate. To be sure, one may be 
more warranted in assuming that an observed granite is of 
ancient formation, or that a leucite-bearing lava is of Tertiary 
age, than one would be in concluding that a sediment of 
almost any common lithologic character belonged in any par- 
ticular part of the stratigraphic column. This is, however, due 
more to the circumstances of the exposure of an igneous mass 
than to any known connection between age and petrographic 
character. 
Geologically it is of interest to know whether igneous masses 
are extrusive or intrusive. Although an indefinite relative age is 
all that is commonly determinable, there are many instances 
where the chronologic relation between the eruption of certain 
magmas and the deposition of certain sediments can be estab- 
lished. In a given province a certain series of eruptions may be 
visibly connected with a known volcanic center, while other 
igneous rocks of the same regionare clearly independent of that 
center, and may be demonstrably very different from its products 
in age. 
When any of these, or of other, geologically significant rela- 
tions of igneous rock masses may be determined, and it is prac- 
ticable to express them on a geological map, such representation 
should certainly be attempted. If such relations are ignored the 
map fails to satisfy the legitimate demands of the geologist for the 
same reason that a map of sedimentary ‘‘lithologic individuals,” 
fails of its full usefulness. 
Metamorphic rocks have been derived from all kinds of 
igneous and sedimentary rocks by a great variety of processes. 
In numerous cases the derivation of the metamorphic product is 
