GEOLOGY: ATWOOD AND MATHER 
177 
The surfaces and s'l are in the relation of a transformation D^. 
Let S be the surface of centers of the spheres of this transformation and 
5*0 the deform of S. Appl3dng the above method to these surfaces, we 
find that the lines joining 0 to the points of Sq are parallel to the nor- 
mals to S. Moreover, the surfaces S and 6*0 correspond with parallelism 
of tangent planes; likewise So and S. 
When c=j=0 in (4), the surface S'l is an iso thermic surface in the rela- 
tion of a transformation with S. 
*Bianchi, Geometria Diferenziale, 2, 88. 
n, c, p. 117. 
'Eisenhart, Differential Geometry, 122. 
^ Theorie Generale des Surfaces, 2, 383. 
^Darboux, Ann. Ecole Normale Supirieure, ser. 3, 16 (1899). 
^Roma, Rend. Acc. Lincei, ser. 5, 24, 161 (1915). 
Ann. Mat., Milatio, ser. 3, 12, 19 (1906). 
GEOGRAPHIC HISTORY OF THE SAN JUAN MOUNTAINS 
SINCE THE CLOSE OF THE MESOZOIC ERA 
By Wallace W. Atwood and Kirtley F. Mather 
GEOLOGICAL MUSEUM. HARVARD UNIVERSITY 
Received by the Academy, February 10, 1916 
The San Juan Mountains are located in the southwestern corner of 
Colorado. From their lofty summits one may look far off into Utah, 
Arizona, and New Mexico. The highest peak in the range, Uncom- 
pahgre, rises to an elevation of 14,300 feet above sea level. There are 
many other peaks whose summits are above 14,000 feet in elevation, 
and a large portion of the mountain area is more than 12,000 feet above 
sea level. 
The core of the San Juan Mountains is composed of a complex series 
of pre-Cambrian gneisses, schists, quartzites, and granites, all of which 
have been more or less affected by intrusive bodies of igneous rocks. 
After this pre-Cambrian complex was so eroded that the relief in the 
region was low, the entire district was submerged and a great series of 
Paleozoic and Mesozoic strata was deposited over the once mountainous 
area. 
At the close of the Mesozoic era and the opening of the Cenozoic 
era there were mountain-making movements which affected the entire 
Rocky Mountain province of North America, and the great dome 
which was then formed in the San Juan area was at once subjected to 
vigorous erosion. As the mountain mass rose erosion began, and as 
the great dome was more and more deeply dissected a mountain topog- 
