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marine platform of varying width encircles the island, beyond which 

 the water again deepens. Submarine profiles indicate that the island 

 began its history as an orographic block, tilted to the north, and form- 

 ing a part of the mainland. It stood 2000 to 3000 feet higher than 

 at present. Following the tilting came the intrusions of porphyrite 

 and diorite. Erosion made considerable progress upon this tilted 

 block, and toward the close of this period the andesitic flows were 

 erupted, accompanied by a slow subsidence. Santa Catalina became 

 an island, depressed, at the close of the downward movement, 1400 to 

 1600 feet below its present level. During the Miocene a long period 

 of erosion reduced the unsubmerged portions to a peneplain. A 

 gradual elevation of 1850 feet followed, with at least one pause in the 

 movement. The last oscillation is exhibited iri the present period of 

 rapid sinking. 



The discussion and exposition (very inadequately summarized in a 

 review of this length) is in general admirable. Exception might, 

 however, be taken to the statement, made twice within the paper, that 

 the shortening of a stream's course by the drowning of its lower 

 ^reaches will cause it to cut down into its alluvial fan. 



F. L. Ransome. 



Geology of the Castle Mountain Mining District, Montaiia. By W. 

 H. Weed and L. V. Pirsson. Bull. U. S. Geol. Surv., No. 

 139, pp. 164, 7 plates. Washington, 1896. 



Recently considerable study has been directed to the isolated 

 -mountains which form the foothills of the Rockies. Such mountain 

 masses offer an inviting field, since they are usually much simpler 

 than the main ranges, and by working out in detail the history of such 

 independent centers of eruption the general order and, perhaps, the 

 causes of differentiation in rock magmas seem likely to be easiest 

 learned. 



The Castle Mountain is a dissected volcano, now rising about 3600 

 feet above the surrounding plain, itself having an altitude of about 

 5000 feet. The mountain mass is about ten miles in diameter, and 

 stands in central Montana between the Little and Big Belt Moun- 

 tains. The stratified rocks of the region include representatives of 

 the Algonkian, Cambrian, Silurian, Devonian, Carboniferous, Jurassic, 

 and Cretaceous, preceding the eruption, and certain Neocene lake 



