Geology and Mineralogy. 273 



height of five hundred feet or more. Such features are seen 

 descending from the Mexican plateaus (of 8,000 feet in altitude) to 

 the Gulf of Mexico. The valleys end abruptly in amphitheaters 

 indenting the floors of the tablelands and dissecting them. 



In the drowned Antillean valleys, long reaches have been dis- 

 covered with slopes of only a foot per mile, like that of the 

 Mississippi, or of some plateau valley. These are separated by 

 abrupt steps, similar to the succession of those descending from 

 the margins of the Mexican tablelands. This point of analogy 

 between drowned and land valleys, as well as the occurrence of 

 short amphitheaters indenting the edges of the submarine plateaus, 

 when carefully compared, very greatly strengthens the conclusions 

 drawn in the "Reconstruction of the Antillean Continent" — 

 namely that the valleys traversing the submarine Antillean 

 plateaus were of land origin, and indicate the depth to which the 

 West Indian continent has sunk, even to a depth of two miles or 

 more. 



3. Die Eruptwgesteine des Kristianiagebletes, Part III; Das 

 Ganggefolye des Laurdalits ; by W. C. B rugger. (Videnskab. 

 Skrift. I Math. Natur. Klasse, 1897, No. 6, Kristiania, 8°, pp. 376.) 

 — The already classic work, which Prof. Brogger has been 

 conducting for many years, upon the igneous rocks of southern 

 Norway, is well known to all penological geologists. The appear- 

 ance of the present important -volume, giving as it does the 

 results of much careful observation and study both in the field 

 and in the laboratory, will be greeted with the greatest interest 

 and attention by all who are interested in the many problems 

 connected with igneous rocks. The special phase of the region 

 which is handled in this memoir is the nephelite-syenite and the 

 attendant varied and peculiar dike rocks which accompany it. 

 Of the many subjects which are discussed it is only possible in 

 this brief notice to summarize the more important. 



There is given first a full description of the nephelite-syenite, 

 which receives the varietal name of laurdaUte. This is character- 

 ized by an unusual amount of lime and magnesia. It is accom- 

 panied by masses of pulaskite, a rather acid type, low in alkalies 

 and poor in nephelite, and by mica-syenite. All of these are 

 regarded as differentiation products of the normal augite-syenite 

 (laurvikite) of the region. 



The accompanying dike rocks are divided into two main 

 classes: (a) the dark colored, generally basaltic types, rich in 

 ferro-magnesian components (lamprophyres) for which the author 

 proposes the name of melanocratic rocks (/uAas, dark and Kpareivto 

 rule) ; and (b) the light colored types of which the feldspars are 

 as a rule the chief minerals and of which the aplites are a repre- 

 sentative class; for these the name of leucocratic rocks (Xcvkos 

 white) is suggested. (It appears to the reviewer that these terms 

 would have had a greater use and precision if they had been con- 

 structed as nouns instead of adjectives, since the terms they are 

 proposed to replace are nouns, i. e. direct names of objects.) 



