384 REVIEWS OF BOOKS. Oct. 



shows a more uniform rainfall than either England or Scotland. 

 Only in a small district around Dublin is the rainfall below 30 

 inches, — indeed, it does not go below 29 inches. As regards the 

 rainfall of the 24 years from 1860 to 1883, there was a 

 comparatively large average amount for the second half of the period 

 over nearly the whole of the British Islands. In the more strictly 

 eastern districts, in the means of the 12 years ending 1883, the 

 excess was generally from 5 to 10 per cent, greater than the means 

 for the 24 years. 



RAPIDS AND WATERFALLS. 



The Scottish Geographical Magazine for Sejjteiiibcr. Edinburgh : 

 Scottish Geographical Society. 



Mr. George G. Chisholm discourses at some length in this number 

 on Eapids and Waterfalls. The great river systems of the world 

 are passed under review to show that geological formation and such 

 breaks in the course of rivers have an intimate connection. Thus 

 in South America, granite, mica, slate, and gneiss rocks predominate 

 on the high plateaux which form so great a portion of its surface, so 

 its magnificent streams are useless as waterways for any extensive 

 commerce owing to rapids and waterfalls. An alternation of such 

 rocks as hard limestones with soft shales as at Niagara is a favour- 

 able condition for a waterfall. The softer rocks being soon worn 

 away by the abrading power of the water, the harder limestone is 

 undermined, and gives way in time at the joints which intersect its 

 surface. In the falls of St. Anthony on the Mississippi, a sandstone 

 of an extremely crumbling texture underlies a schistose limestone 

 only 15 or 20 feet in thickness, and consequently the recession has 

 been more rapid than at Niagara, indeed more than five times that 

 calculated by Lyell. Instances of the like character, where the 

 mountain limestone has shale below it, may be seen at Ashgill 

 Force on the Ashgill, a tributary of the Tyne ; and in the course of 

 the Tees, the Swale, the Yore, and the Wharfe, and many other 

 streams flowing more or less east to west from the backbone of 

 England. In the higher course of the Ericht above Blairgowrie in 

 Perthshire, waterfalls caused by the conglomerate or plum-pudding 

 stone below harder sandstones being most quickly abraded by the 

 current, form a type peculiar to the Old Eed Sandstone formation. 

 But other features in the physical formation of a country, such as 

 trap dykes or dislocations, also facilitate the formation of rapids 

 and waterfalls. Given the conjunction of hard with soft rocks, as 

 well as the combination of cracks and crevices into which the 



