Dr. J. W. Spencer — The Yumuri VaUey of Cuba. 501 



recent canons along the streams in Cuba. After seriously con- 

 sidering the untenable hypothesis of the origin of the basin as due 

 to solution, I found that the explanation of the basin with modern 

 insignificant outlet was due to recent dislocation, as was shown in 

 the fault exposed in the longitudinal section of the cafion. The 



Fig. 2. Longitudinal section through the canon of the Yumuri, at right angles 



to the strike ; S, sea-level ; C, raised coral-reefs ; M, Matanzas limestones ; 



B, slight unconformity between Miocene limestones and Miocene sands ; D, base 



of Miocene ; A, undulations in Eocene limestones ; F, fault whose dip is 



■ 60°, general dip of strata varies from 20° to 30°. 



Matanzas limestones have a thickness of about 150 feet; the 

 Miocene 790 feet; and the Eocene above the fault 760 feet, but 

 after making allowance for the dislocation along the fault, the total 

 thickness of the Eocene may reach to 1200 or 1500 feet in the 

 section along the side of the valley beyond the limit of Figure 2. 

 The vertical elevation produced by the fault varies from 250 to 

 nearly 400 feet, as shown in Figure 3. The valley had formerly 

 extended in two lobes round an island to the Matanzas Bay, bat 



Fig. 3. Section across the end of Yumuri Valley (same as section A B, Fig. 1). 

 Broken shading represents the barrier raised in front of the valley ; A and B, 

 the former extensions of the two lobes of the valley ; and C, the site of the 

 canon. The broken shaded section is about three miles long, and the maximum 

 height about 450 feet. 



with the elevation of the ridge to the named height the basin was 

 produced. The plain of uplift has been preserved and exposed in 

 the fault, shown at the inner end of the carion, or on the side of the 

 barrier-ridge facing the basin. The plain of the fault dips 60° with 

 the adjacent strata crushed, the only structure of the kind seen in 

 the whole section. 



The discovery of the fault settled the origin of the basin, and the 

 occurrence of late Pliocene beds in the uplifted mass brings down 

 the date of the dislocation, after the erosion of the valley, into the 

 later part of the Pleistocene period, or later, as would be suggested 

 by the newness of the walls of the canon. 



After the earlier Pliocene elevation, there was a subsidence 

 somewhat below the present altitude of the land, when the Zapata 

 (mid-Pleistocene) formation was deposited.^ At other places in 

 Cuba these accumulations obstructed the drainage of the earlier 

 Pleistocene valleys, and caused streams to cut across the last made 



^ This formation was described at the same time as the Matanzas. 



