POSITIONS OF ANCIENT CONTINENTS 375 
We have seen that the latitudinal extension of the Pre-Cambrian 
continents was preserved with remarkable persistence through 
Paleozoic time. The series of charts given in another connection 
show at a glance this persistent character of the large primeval 
continental masses. The chart of Late Triassic time, still clearly 
brings out the same original latitudinal direction of the continents. 
In the Jurassic period, however, the Indian Ocean became definite¬ 
ly established by the foundering of a large portion of eastern Gond- 
wana; and in Tertic time the Atlantic Ocean finally extended south¬ 
ward between Africa and South America and connected with the 
Antarctic Ocean. As a result of these fracturings of the old con¬ 
tinental masses the continents of the present day with their pre¬ 
dominating longitudinal extension were formed. 
It would, then, seem that the Pre-Cambrian continental outlines 
are lost in the present configuration of the surface of the earth. 
Nevertheless there is good evidence that the old boundary lines of 
the original Pre-Cambrian continents continue to exist as distinct 
features in the framework of the earth. 
A comparison of the original “central massives,'' or nuclei, and 
also the present mobile, or earthquake, tracts (together with the 
tracts of Tertic folding), with charts of the Pre-Cambrian and 
Paleozoic continents brings out readily the fact that these mobile 
tracts, in a most remarkable manner, pass along the supposed 
boundaries between the Pre-Cambrian continents as we have es¬ 
tablished them by the Pre-Cambrian fold directions and other evi¬ 
dence. This is especially clear in the case of the boundary be¬ 
tween Eurasia and Gondwana. 
The earthquake tracts distinctly form a latitudinal belt (also 
known as “Libbey’s circle”) around the earth, separating North 
and South America, traversing the middle Atlantic and separating 
first Eurasia from Africa and then from the other Gondwana ele¬ 
ments (India, Arabia and western Australia) and reaching again 
across the middle Pacific to Central America. 
Another belt follows the eastern, northern and western bound¬ 
aries of the Pacific Ocean. It seems to be completed by the Ant¬ 
arctic seismic regions. There are further known independent 
shorter longitudinal tracts in the Atlantic and Indian oceans, that 
indicate that the forces active along the later zones of fracturing 
