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



stances quite different from, and in a few cases directly op- 

 posed to those of Dr. Florentino Ameghino and his brother 

 Carlos Ameghino ; yet it is believed that most of the conclu- 

 sions reached are fully warranted by the facts observed ; and 

 that in the present paper there will be found an important 

 supplement to our knowledge of this region, which has already 

 been so much increased by the combined efforts of the brothers 

 Ameghino. 



Mesozoic Rocks. 



Jurassic? — The oldest sedimentary deposits seen by the 

 writer were a series of black, very hard, but much fractured 

 slates, with Ammonites fairly abundant, bat not sufficiently 

 well-preserved to admit of identification. These beds, which 

 I propose to call the Mayer River beds, in some places at least, 

 rest directly on the eruptive rocks which here form the great 

 mass of the Cordilleras ; they are well represented on the right 

 bank of the lower fork of Mayer "River, just where it emerges 

 from a deep gorge about three miles above its confluence with 

 the main stream.* A greater development of these beds may 

 be seen at the west end of Bald Mountain, an elevation in the 

 middle of Mayer Basin ; at this locality they have a decided 

 eastwardly dip and an estimated thickness of 1500 feet. In 

 their uppermost layers, they are sometimes of a red or yellow 

 color and are less fractured and more cleavable than on Mayer 

 River. No fossils were found on Bald Mountain. 



The Mayer River beds are referred to the Jurassic, partly 

 because of the Ammonites found in them, which appear to 

 resemble Jurassic forms ; but more especially on account of 

 their lithological characters and because of the great thickness 

 of the sedimentary rocks overlying them, which, by the pre- 

 sence of Dinosaurian remains in their uppermost strata, can 

 hardly be more recent than Cretaceous. 



Cretaceous. — Immediately, but unconformably, overlying the 

 Mayer River beds, is a series of heavily bedded, light brown 

 sandstones, becoming variegated above, exceedingly barren of 

 fossils and with an estimated thickness of 1000 feet. In their 

 lower layers they resemble in appearance the Dakota sand- 

 stones of our western States. They are well represented near 

 the source of Mayer River, where they extend for several 

 miles in an unbroken wall, forming the southern border of 

 Mayer basin. With the exception of uncharacteristic plant 

 impressions no fossils were found in these sandstones. They 

 are referred to the Cretaceous upon stratigraphical evidences 



* Many of the water courses and topographic features mentioned in this paper 

 will not be found located on any of the current maps of Patagonia, For refer- 

 ence they have been located and given names on the accompanying map. 



