10 Windhausen — The Cretaceous- Tertiary 



to such portions of basal structures as for instance the 

 widespread sheets of Patagonian quartz porphyry, inves- 

 tigations justifying any pronouncement as to origin and 

 age are lacking.- 7 



At many points, especially in Northern Patagonia, the 

 leveling of the old base line and the sedimentation of the 

 continental Variegated Sandstones of the Upper Creta- 

 ceous are the events preceding the main Atlantic 

 transgressions, which on this surface laid down their 

 sediments. In other cases, the extension of the trans- 

 gressions was obviously dependent on ancient orographic 

 elements of Paleozoic structure (now partly in the under- 

 ground). The estuary of the La Plata River formed the 

 Neogene gateway of marine invasion. It will be proved 

 in Part III that the northern limit of the San Jorge- 

 Sea was formed by the socle (or floor) of the Sierras of 

 Buenos Aires, further by an old granitic mass in the 

 heart of the Pampa ; also that there was renewed activity 

 along the ancient tectonic line between the Precordilleran 

 elements and the mass of Pampine Sierras, forming a 

 large depression, in which brackish sediments were 

 deposited. Furthermore, it is extraordinarily important 

 that the line of the Rio Negro-valley, where the San 

 Jorge-transgression entered by a northern branch, has 

 the character of a tectonic chief line, being the limit of 

 the old mass in the underground of Patagonia. 



Obviously, the oldest of these marine transgressions 

 from the Atlantic basin is of special interest, as it marks 



27 It is not the aim of this paper to go into a discussion of character and 

 age of the old root forming the foundation of Cretaceous and Tertiary 

 formations of Patagonia. F. Ameghino (Les formations sedimentaires, 

 pp. 35 etc., fig. 3) gave in 1906 a sketch of the then known exposures of 

 old rocks in the Patagonian mesetas, and recently E. Wichmann gave for 

 the First Congress of Natural Science in Tueuman (November, 1916) a 

 brief review on the same subject, with special reference to Northern 

 Patagonia. The chief components of this old basement are gneiss, mica 

 schists, phyllites and grauwaekes, strongly inclined and folded and with 

 transverse cleavage, showing also intrusions of granitic rocks, frequently 

 accompanied by injections of quartz. Upon the base-leveled surface of 

 these old rocks rest large sheets of quartzporphyry and generally the 

 exposures of the latter are more numerous than those of the older rocks. 

 This basement comes to the light as window-like apertures among the 

 Cretaceous and Tertiary strata. There are reasons for the assumption 

 that faults of a considerable degree cross this ancient platform. Its age 

 is uncertain, but there is a great similarity to the Caledonian or Hereynian 

 .structure of the so-called ' ' Pampine Sierras ' ' of Central Argentine. In 

 the map accompanying this paper, the old rocks of the continental platform 

 and the quartzporphyry sheets are comprised under the same symbol, all 

 outcrops being purposely enlarged. 



