274 MR. J. A. DOUGLAS ON GEOLOGICAL SECTIOK"S [vol. Ixxvii, 



direction (towards the exterior of the chain) or b}'' the ' pinehing- 

 in ' of steepl}^ inclined folds and the consequent preservation of 

 younger beds among those of earlier date over which they were 

 originally transgressive. 



lii the neighbourhood of Tarma the above-described Mesozoic 

 rocks rest unconformably on a series of strongljr-folded olive-green 

 and red shales, which are assigned by Steinmann to the upper part 

 of the Lower Silurian. They are, however, quite distinct from the 

 black, graptolite-bearing, Ordovician shales of the Carabaya 

 district of Southern Peru, and in lithological character they more 

 closely resemble the Devonian rocks of the region north of Lake 

 Titicaca. Since they aj)pear to contain no fossils, 1 have deemed 

 it inadvisable, however, to assert a definite opinion as to their age, 

 and have recorded them here provisionally as Upper Palaeozoic. 



Near the village of Acobamba these shales rest with apparent 

 conformity upon a series of limestones, quartzites, and conglo- 

 merates. These in turn are succeeded by phyllites, which, when 

 traced towards the east, become increasingly metamorphosed until 

 they assume the character of true mica- schists. 



In the midst of this metamorpliic series, a short distance west 

 of Palca, occur black crystalline limestones ap])arently but little 

 altered. Their presence here, according to Prof. Steinmann, may 

 be accounted for by means of a steeplj^-inclined thrust-plane along 

 which the older rocks have been carried over them from the west. 

 It is possible, however, that thej^ occur in a normal succession, 

 and have remained comparatively unaltered ; while the less-resistant 

 argillaceous shales, associated with them, have been converted into 

 phyllites. 



The age of these limestones is b}^ no means certain. Steinmann 

 assigns them to the Carboniferous, from the reputed discovery in 

 the neighbourhood of Lonsdalea and Procluctus. The locality 

 from which were derived the specimens that led him to form 

 this opinion is, however, unknown, for they were presented to him 

 during the course of his journey, without, it appears, any definite 

 record of their origin. While it is possible that his deduction is 

 correct, it is regrettable that a note explaining the exact value of 

 such evidence was not inserted in his account of the district. 

 Between Palca and Carpapata the steep sides of the valley are 

 formed almost entirely of granite, veined with both large and 

 small intrusions of porphjay, and as we approach Huacapistana 

 this is seen to be replaced once more by metamorphosed sediments 

 in the form of mica-schists. 



Another and more extensive outcrop of granite is met with 

 immediately below Utcuyacu, Avhere it exhibits proof of having 

 been subjected to intense dynamic crushing, with the consequent 

 development of typical gneissic structure. It is important to note 

 that this is a white rock, quite distinct, not only in lithological 

 character, but also probably in geological age, from the red granite 

 of the Perene district, to be described below. It is clearly of later 

 date than the surrounding strata, for its intrusive character can be 



