254 proceedings: geological society 



MgO. He inquired as to the mineral composition, remarking that 

 with such a large amount of alkalies, alkaline pyroxenes should be ex- 

 pected. ScHRADER had no description of the mineral composition at 

 hand. G. F. Loughlin spoke of the difference between the specimen 

 of quartz monzonite exhibited and the Leadville porphyry. The Arizona 

 specimen appeared more like the quartz monzonites of Utah. He also 

 called attention to the chemical affinities between the analyses of 

 gabbro and quartz monzonite shown, and thought it quite probable 

 that the ores might have been derived from the gabbro. 



F. E. Matthes. Studies on glacial cirques in the Sierra Nevada. 

 The better preserved cirques of the central High Sierra, especially those 

 hewn in fairly massive igneous rocks, are found to consist of two dis- 

 tinct and contrasting parts, a lower bowl with smooth, concave slopes, 

 in many instances holding a tarn, and an upper chff rising from the 

 periphery of the bowl, with rough, hackly face, arid nearly vertical 

 profile. The relative proportions of bowl and cliff are by no means 

 the same in all cirques; but the bowl is in every case the dominant fea- 

 ture, constituting the main part of the cirque, while the cliff is distinctly 

 accessory in character, although occasionally assuming considerable 

 height. 



The riven, hackly face of the cliff clearly bespeaks frost action. 

 The verticahty of its profile and the tendency to overhang, moreover, 

 point to basal sapping; that is, they show that the locus of most intense 

 frost action is situated at the base of the wall. The conspicuous smooth- 

 ness of contour and the striated and polished surface of the concave 

 bowl-slopes, on the other hand, attest corrasion, principally abrasion — 

 by moving ice. This abraded appearance sets in immediately below 

 the base of the cliff, showing that at that line frost action ceases abruptly 

 and that there is no transition zone in which the two processes blend 

 their effects. The Imse of the chff accordingly constitutes a well-de- 

 fined and conspicuous boundary line, which is interpreted as indicating 

 the depth to which the bergschrund of the ancient glacier opened and 

 to which frost action was permitted to penetrate. (Willard Johnson 

 and Gilbert have referred to it as the "sap line.") If this interpreta- 

 tion is correct, it necessarily follows that the entire cirque bowl — the 

 major part of the cirque — is elaborated through the corrasive action of 

 the ice masses contained in it. Mass corrasion, accordingly, would 

 appear to be the dominant cirque sculpturing process, while bergschrund 

 sapping, as T. C. Chamberlin and R. D. Salisbury have suggested, 

 is merely a peripherally working, auxiliary process. The two processes, 

 while associated, do not necessarily work at the same rate. In many 

 instances cliff sapping appears to have outstripped mass corrasion, l)e- 

 cause it was aided by the development of joints in the upper parts of 

 the rock mass through weathering at the preglacial surface of erosion. 

 In such cases the cliff stands back from the bowl rim and is separated 

 from it by a narrow shelf. Abrasion by snow shod with rocks fallen 

 from the chff smooths the shelf and transforms the bowl rim into a 

 rounding curve. 



