338 J. B. Hatcher — Geology of Southern Patagonia. 



introduced, and the entire aspect of the fauna changed from 

 Eocene to Miocene, according to Dr. Ortmann, who has also 

 studied our collections of invertebrates from these beds and 

 has identified the following forms : Cldaris sp. ? ; Scutella sp. ? ; 

 Bryozoa ; Terebratula patagonica (Sow.); Ostrea phillippii 

 (Ort.) ; 0. hatcheri (Ort.) ; Peotunculus sp. ? ; Glycimeris sp. ? ; 

 Fissiorella; Solarium; Trochita costellata (Phil.) ; Turritella 

 affinis (Hup.); Crepidula gregaria (Sow. ?) ; Scalaria rugolosa 

 (Sow.) ; Struthiolaria chilensis; Natioa solida (Sow.); Balanus 

 varians (Sow.) ; Chthamalus antiquus (Phil.). 



The Supra-Patagonian beds are composed of alternating 

 layers of sandstones and clays, usually of a yellow or light 

 brown color with a rich invertebrate fauna. Ameghino states 

 that they have a thickness of 30 meters, but in the interior, 

 along the base of the Cordilleras, they certainly attain to a 

 much greater thickness, and I should not hesitate to allot to 

 them a thickness of fully 150 meters at Shell Gap, where Lig- 

 nite Creek emerges from Mayer basin. In this region, as also 

 on the upper Rio Chalia, they rest unconformably upon the 

 Guaranitic beds and dip to the eastward at an angle of about 

 15° as shown in fig. 3. 



J £ joo/f. £ 20c eft. J) /CGOft- £ wfan'/maW f 



Fig. 3. Section of sedimentary deposits as displayed on south side of Lignite 

 Creek and southern border of Mayer basin, from a point one mile east of Shell 

 Gap to the western border of the basin. A-B. Fresh-water, Santa Cruz beds • 

 B-C. Marine Supra-Patagonian beds; C-D. Fresh- water? Guaranitic beds; 

 D-E Barren sandstones : E-F. Marine Mayer River beds and igneous rocks. 

 Distance from A to F about 15 miles. Relative inclination of strata to base-lme. 

 A-F. exaggerated for effect, thus increasing thickness of deposits relatively to 

 length of section displayed. 



The Santa Cruz beds. — I cannot agree with Dr. Ameghino 

 in considering the Santa Cruz beds as belonging to the same 

 series with the Supra-Patagonian beds. My reasons for sepa- 

 rating them are because the Santa Cruz beds are of fresh 

 or brackish water origin, as shown by the diatoms which 

 they contain ;* and also I am brought to this conclusion by 

 the fact that all along the foot hills of the Cordilleras the 

 Supra-Patagonian beds were observed, inclined at high angles, 

 while in the same region the Santa Cruz beds are approxi- 

 mately horizontal and show almost no evidences of disturbance. 

 At Shell Gap, on Lignite Creek, this creek has cut a narrow 

 gorge through the sandstones and clays of the Supra-Pata- 

 *See Geol. Obs. on S. A., by Darwin, p. 117. 



