574 Revieivs — Papers by Professor W. M. Davis. 



Three series of igneous dykes occur in the schists and quartzites 

 of the north-west of Ireland; those belonging to the series inter- 

 mediate in age are described in the Survey publications as 'felstones,' 

 but they have since been shown by Hyland and others to include 

 various rocks of the lamprophyre group (camptonite, vogesite, 

 minette, kersantite). In the second paper quoted above these 

 determinations are confirmed, and a glassy olivine-basalt belonging 

 to the same series is also described. 



IV. — Some Eecent Papers by Professor W. M. Davis. 



WHEN ascending the valley of the Ticino on the way to 

 St. Gothard, Professor W. M. Davis observed that the side 

 valleys opened into the main valley several hundred feet up, and 

 streams cascaded down in sharp-cut shallow clefts. Such dis- 

 cordance between main and lateral valleys appears to be the rule 

 in Alpine regions, and in all valleys which have been strongly 

 glaciated ; and the attention which Professor Davis has given 

 to the subject dispelled the doubts which he had long felt as 

 to the ability of ice to erode deep . valleys and basins. It is not 

 surprising to learn that the features are, in his opinion, of so much 

 importance that a special name, that of ' hanging valleys,' suggested 

 by Professor Gilbert, is employed to designate these side valleys. 

 The peculiar features to which he draws attention are ascribed to 

 the action of the heavy ice-stream that once filled the valley to 

 a great depth : this trunk glacier at times rose so high that the 

 small lateral glaciers were held back and prevented from deepening 

 their channels, while the main valley was continuously subject to 

 erosion. (" Appalachia," vol. ix, March, 1900.) 



The recognition of the competence of glaciers thus to deepen and 

 widen their valleys is regarded by Professor Davis as an important 

 supplement to the belief that they can excavate lake basins. He 

 therefore pursues the subject of " overdeepened main valleys and 

 hanging lateral valleys," in an article on Glacial Erosion in France, 

 Switzerland, and Norway (Proc. Boston Soc. Nat. Hist., vol. xxix, 

 July, 1900). He points out that if the existing breadth of the 

 glaciated valleys, which are broad-floored, had been acquired in 

 the ordinary manner by the lateral swinging of the main stream 

 and by the weathering of the walls, the long time required for such 

 a change would have amply sufficed for the side streams to cut down 

 their valleys to grade with the main valley. He concludes that the 

 troughs were deepened and widened by ice-action. Similar features 

 occur on the borders of Lake Lugano and other Alpine lakes, in our 

 own Lake District, and in Norway. Irregularities along the main 

 valleys eroded by glaciers may have arisen through the greater 

 destruction of softer or more jointed rocks ; and if the glacier 

 should vanish by climatic change while in this condition a lake 

 would occupy the deepened reach, and its outlet would flow forward 

 over rocky ledges to the next lower reach or lake. As the energy 



