Ovigiii of Pecimatlfe. — Crosby and Fuller. 179 
tary rocks; but, following Iddings and other recent writers, 
it must be admitted also for the igneous rocks. A dike is a 
more or less highly differentiated portion of some deep-seated 
magma, and it is clearly newer than the walls. An ascen- 
sionist, certainly, can say nothing more for a vein, save that 
while in the formation of the dike heat is the chief agent 
with water cooperating (aqueo-igneous fusion), in the forma- 
tion of the vein water is the chief agent witli heat cooperating 
(igneo-aqueous solution). The greater mobility of the so- 
lution permits a higher degree of concentration; but concen- 
tration is a universal process and participates in the forma- 
tion of eruptive masses and sedimentary deposits as well as 
of veins. 
In the opinion of the writers, no sharp line of d(Mnarcation 
can be drawn between dikes and veins, and veins are clearlj- 
entitled to some degree of recognition in thelithological class- 
ification. In a broad view of the early history of the earth, 
all the sedimentary and vein rocks are, of course, secondary 
with reference to the primitive igneous crust, but so are the 
igneous rocks with which we are now acquainted. Probably 
none of the igneous rocks which have been studied are truly 
j)rimitive, and their derivation in some cases from sediments 
is claimed by many able observers. 
Heat and water are the two great agents concerned in the 
modification and differentiation of the eartirs crust ; and they 
are generally, if not everywhere and always, in cooperation, 
heat prevailing, as a rule, at great depths, and water at the 
surface. Thus arise the two principal classes of rocks — ig- 
neous and aqueous. In either class the rocks may be deep- 
seated (newer than both walls) or supei'icia) (newer than one 
wall), in other words, intrusive or contemporaneous. The 
intrusive igneous masses are known as dikes, etc., and the 
contemporaneous as lava-flows ; while the intrusive aqueous 
masses are known as veins, etc., and the contemporaneous as 
sedimentary strata. Between the vein rocks and dike or plu- 
tonic rocks we have, at great depths, a perfect transition, in 
the pegmatites; but between volcanic rocks and sediments 
transition forms are wanting, because the limited pressure 
does not permit the cooperation of heat and water. 
