12 STUDIES IN GEOLOGY, No. 3 



it is cut off by the transverse fault mentioned, and the 

 Lower Zorritos is juxtaposed to the Variegated. From the 

 ridge at the point where the three formations meet, the whole 

 situation is clear. 



The principal drainage of the region is determined by the 

 fault-lines, and the rough physiographic aspect of the terrane 

 is due to the irregular disposition of the fault-blocks, which 

 form the hills between the drainage channels. 



ANALYSIS OF THE FAUNA. 

 I GENERAL CONSIDERATIONS. 



An analysis of the collection here studied, made from 

 either the biologic or the geologic viewpoint, can present 

 evidence of positive nature almost exclusively. It is almost 

 certain that enough of the entire Zorritos fauna is as yet 

 unknown to forbid the drawing of conclusions based on the 

 absence of any element, and although much can be deduced 

 from the presence of those forms now known, the satisfy- 

 ing completeness of a study which includes all inferences, 

 positive and negative, must be foregone until highly detailed 

 collecting either furnishes elements yet to be known or 

 proves that the majority of the species are embodied in the 

 present study. In the case of the Upper Zorritos formation 

 it is not certain that a very large fauna exists, although 

 it is probable that considerable additions may be made 

 through protracted search for new localities, but both the 

 Variegated and the Lower Zorritos formation give promise, 

 through a multitude of indeterminate fragments in the 

 matrix of the present collection, of a fauna which may 

 approach in richness some of the famed Antillean and Flor- 

 idian assemblages of approximately similar age. 



One difficulty which must be solved before the study of 

 these faunas can be completed is the procuring of good 

 material. This applies in particular to the minute and more 

 delicate species. The matrix in which most of the specimens 



