part 1] GEOLOGICAL SECTIONS THROUGH THE ANDES. 31 



associated dykes, which formerly cropped out farther west, but are 

 now completely concealed by the Tertiary volcanic covering. 



The diorite of Maravillas, described below, to which they bear 

 some resemblance, breaks through the limestones, and is obviously 

 an intrusion of post-Cretaceous age. 



The red marls with which the limestones are associated are very 

 similar in appearance to the Cretaceous marls of the Coro Coro 

 district of Bolivia, and the thinning-out of the limestones when 

 followed in this direction shows that, while the country in the north 

 gradually became depressed beneath the waters of the Cretaceous 

 sea, shallow-water conditions continued to prevail in the south. 



The conglomerates and overlying limestones undulate but slightly 

 in the neighbourhood of Lagunillas. Between the two lakes, how- 

 ever, they show indications of being much faulted, and dip steeply 

 until they reach the valley-floor, the conglomerate attaining its 

 greatest development in the rugged cliffs bounding the eastern 

 basin. 



The physiography of the district is also of considerable interest, 

 and it is a matter of some difficulty to account for the origin of 

 these twin lakes, separated by a steep though narrow neck of land. 

 The most jirobable solution is that they owe their position to 

 faulting on a large scale. 



Numerous instances of minor faults, affecting both the 

 Cretaceous and the underlying rocks, are, indeed, visible along 

 the shores of Lagunillas, where they are occasionally associated 

 with igneous intrusions in the form of small basaltic dykes. 

 Although these seem to bear little relation to the present contour 

 ■of the lakes, they at least show that the district was subject to 

 dislocation and subsidence, and in this way relief may have been 

 •obtained from the earth-stresses which a few miles away to the east 

 have caused pronounced folding in the Cretaceous limestones. 



Beneath the basal conglomerates of this series lies an older 

 system of rocks, which are well exposed on the promontories of 

 the lake-shore, where they consist of false-bedded white and yellow 

 quartzites, frequently containing bands of small white quartz- 

 pebbles. They are strongly folded, but have a general dip of 

 about 45° south-south-westwards (see fig. 3). Most of these beds 

 appear to be unfossiliferous ; but near the base occur black shales 

 and a thickly-bedded, dark-grey, cherty limestone which yielded a 

 scanty and badly-preserved fauna, comprising one or more species 

 of brachiopods (Terebratula), gasteropods {Nerinea), and a 

 lamellibranch (JPerna). These remains were much silicified and 

 difficult to extract, and they seemed to afford little clue as to 

 their age. Mr. S. S. Buckman, however, has kindly examined 

 some of the specimens for me, and reports that the Terebratulas 

 almost certainly belong to the perovalis group, and. as such, indi- 

 cate beds of Inferior Oolite age, discites zone. 1 The fragmentary 



1 Cf. T. perovalis Gottsche non Sowerby, pi. iv, fig - . 9, ' Beitriige zurGeologie 

 •& Palaontologie der Argentinischen Republik ' Palasontographica (1878), 

 Naturgesch. der Vorwelt, Suppl. iii, Lief, ii, Heft 2. 



