482 REVIEWS 



post-Glacial valley diverted the old Niagara above the Whirlpool and con- 

 centrated the flow in such a manner that a deep, narrow gorge was exca- 

 vated. Afterward this was partially filled with blocks of stone and the 

 present Whirlpool Rapids were formed. The basin at the site of the 

 present falls is part of the shallow valley of a former tributary of an old 

 outlet of the Erie basin. The Upper Rapids owe their existence to the 

 fact that the falls, in working back, are climbing the bank of this refilled 

 channel. Below the Whirlpool the well-known Foster Flats indicate the 

 position of the floors of the Niagara River at the time when there were two 

 falls, one in advance of the other. A third cataract existed still farther 

 down the gorge and persisted long after the other two had become united. 

 When the falls had retreated just beyond Foster Flats the drainage from 

 the three upper Great Lakes was added to that of the Erie basin, which had 

 already found an outlet through the Niagara. A study of the fluctuations 

 of the lake levels shows that the tilting of the Great Lakes area has not 

 operated during the last 50 years. The above are only a few of the features 

 connected with Niagara and its history which are discussed in this very 

 complete study of the river. Mr. Spencer places the age of the falls at 

 39,000 years. H. H. 



Transvaal Mines Department. Report of the Geological Survey for 

 the Year 1906. Pretoria, 1907. 140 pp., 37 pi. 

 The plan of the Transvaal Survey is to publish separate sheet maps of 

 definite portions of the Colony, together with descriptions of the geology of 

 these areas. The report for 1906 contains descriptions of the structure, 

 topography, and stratigraphy of nearly 5,000 square miles of territory and 

 pays especial attention to deposits of economic value. Occurrences of 

 magnesite, coal, magnetite, hematite, antimony, and gold are cited. In 

 connection with the non-detrital auriferous deposits in the Lyndenburg and 

 Carolina districts it is of interest to note that they are of the bedded ore- 

 sheet type: water with gold in solution traveled along bedding planes until 

 the precious metal was precipitated by ferrous compounds. Many of the 

 illustrations in this volume will be appreciated by those who delight in 

 grandly wild scenery. H. H. 



Maryland Geological Survey. Vol. VI. 572 pp., 51 pis., 19 figs., 

 map. Baltimore, 1906. 



Part I of this report contains a complete summary of the physical feat- 

 ures of Maryland, describing the physiography, geology, mineral resources, 

 soils, climate, hydrography, terrestrial magnetism, and forestry ^of the 



