536 SO STEP TT MIE AG OINGRE: 
mention and pass on. Some, however, I discuss fully now for 
the first time. 
1. Cycles of constructive forms.—Of the four great kinds of 
earth movements treated of in my address as President of Geo- 
logical Society of America, December 1896," the greatest, viz., 
that by which are formed oceanic basins and continental arches, 
being determined by unequal radial contraction in the secular 
cooling of the earth, which in its turn is the result of an original 
heterogeneity in the density and especially in the conductivity of 
different parts of the earth, must have a cycle coéxtensive with 
the life of the earth itself, and therefore may be left out in this 
discussion. But superimposed on this greatest, there are other 
cycles of oscillations of the earth’s crust over wide areas—of 
continental elevation and depression, accompanied with the for- 
mation of great mountain ranges. These are the cycles of next 
importance, and with which we are here concerned; for they 
determine all other cycles mentioned above, and therefore the 
occurrence of what I call critical periods. They are demon- 
strated by the widespread unconformities which occur at these 
times. Upon these, again, are superimposed still lesser cycles 
which, however, do not concern us here. I have sufficiently 
treated of these, both great and small, elsewhere, and therefore 
pass on. 
2. Cycles of climatic conditions. — Coincidentally with the 
changes by continental elevation and depression with their 
attendant mountain-making, there have undoubtedly been con- 
current changes in climatic conditions of many kinds, especially 
of temperature. These changes were, on the whole, probably 
gradual throughout the era, but culminate and oscillate in the 
critical period which closed it and constituted one of its most 
marked features. The two most conspicuous examples are those 
which occurred at the end of the Paleozoic and at the end of 
the Tertiary. The early Paleozoic was eminently an oceanic 
period. During the whole Paleozoic there was, in this country 
at least, a gradual elevation and enlargement of the continent, 
t Bull. Geol. Soc. Am., Vol. VIII, p. 113, 1897.; Sci., Vol. V, p. 321, 1896. 
