172 O. N. Rood on a Secondary Spectrum. 
(9.) The principle adopted from LeConte, that the bottom of 
a geosynclinal becomes weakened, as subsidence and surface 
sedimentary accumulations go forward, through the access of 
heat from below or the rise of the isogeotherms (the change of 
level in a given isothermal plane having been seven miles in 
the Appalachian region), and that this in an important degree 
has made possible the catastrophe in which synclinoria have 
resulted. 
(10.) That, while igneous eruptions and metamorphism have 
each attended the formation of synclinoria, still in cases where 
the plication was greatest the igneous eruptions have been 
least in amount or absent.; and that the most extensive igneous 
eruptions have taken place on continental borders after the 
crust had become too much stiffened to bend freely before the 
lateral pressure. 
11.) That in the upturning and plication attending moun- 
tain-making, the heat from the transformation of the motion 
was sufficient (in connection with other heat from a rise of the 
isogeotherms due to previous surface accumulations) to cause 
metamorphism; and also the pasty fusion which obliterates 
all stratification and gives origin to granite, and which may fill 
cavities or fissures, and so make veins that have all the aspect 
of true igneous ejections; and, as a more extreme effect, it may 
produce, as Mallet says, the degree of fusion which belongs to 
plastic trachyte, and give rise to trachytic and other ejections 
through fissures or volcanic vents. t— 
_ (12.) That the chief source of igneous rock is the plastic layer 
situated beneath the true crust, or the local fire-seas derived 
from that layer. 
The discussion has enlarged beyond its limits in my previous 
Art. XIX.—On a Secondary Spectrum of very large size, with 4 
Construction for Secondary Spectra ; by Prof. O. N. Roop. 
trum is due to the circumstance, that the spacing of the colors 
in the two original spectra is not accurately correspondent, and 
