7GQ 



SCIENCE. 



[N. S: Vol.. II. No. 47. 



double synclinorium, whose lowest member is 

 the Cambro-Silurian Stockbridge limestone. 

 This is succeeded by the Berkshire schist, the 

 Bellowspipe limestone aud the Greylock schist, 

 all Silurian. Now, the interesting geological 

 thesis established by the monograph is that the 

 metamorphosed elastics of the Hoosac Moun- 

 tain are the shore deposits, which in the case 

 of the Hoosac schist correspond to the deeper 

 water, Stockbridge and Bellowspipe limestones 

 and their accompanying schists. The determi- 

 nation throws a flood of light on the entire 

 stratigraphy of the region, and simplifies the 

 problem of the Green Mountains. The diffi- 

 culties that were overcome in tracing out these 

 metamorphic schists to their original sediments, 

 in proving the uncomformability of the Ver- 

 mont conglomerate gneiss upon the Stamford 

 gneiss, when the foliation was the same in both, 

 and the neat way in which it was done by the 

 discover}^ of the eroded and depressed pre-Cam- 

 brian outcrop of a trap dike in the Stamford 

 gneiss, which was buried under the Vermont 

 formation, all called for patient study and close 

 observation in the highest degree. And when 

 the passage of the Hoosac schists into the Stock- 

 bridge limestone was finally established, a very 

 hard problem was at last solved. The authors 

 are to be warmly complimented and congratu- 

 lated on their success. 



Besides the stratigraphic results, many im- 

 portant contributions are made to our knowl- 

 edge of the general metamorphism of sediments 

 to crystalline schists. 



The three authors were also aided in a de- 

 gree calling for mention by Mr. B. T. Putnam, 

 whose untimely death removed him in the 

 midst of his career, and by Prof W. H. Hobbs. 

 The report is richly illustrated with that pro- 

 fusion of maps and plates which is only attain- 

 able in this country by attaches of the United 

 States Survey. The investigations have been 

 continued on the south by Professor Dale, whose 

 later results are published in the Fourteenth 

 Annual Eeport of the Director as reviewed in 

 these columns, p. 6.32. 



J. F. Kejnip. 



The Laccolitic Mountain groups of Colorado, Utah 

 and Arizona. Whitman Cross. 14th An- 



nual Report of the Dh-ector of the U. S. Geo- 

 logical Survey, Washington, 1894. Pp. 

 165-241. Pt. ii. 



Mr. Cross makes in this paper the second 

 considerable contribution to our knowledge of 

 laccolites, the first having been made by Gilbert 

 in 1877. The West Elk Mountains, in Colo- 

 rado, including Ragged Mount, Mt. Marcel- 

 lina, the Anthracite range, Mt. Axtell, Mt. 

 Carbon, Mt. Wheatstone, Crested Butte, Gothic 

 Mount and probably others in the same group, 

 are laccolitic in origin. So also are the San 

 Miguel Mountains, about 70 southwest of the 

 West Elk group. . Still farther south, at a dis- 

 tance of 25 miles, the La Plata Mountains fonn 

 a remarkable group of laccolites. About 65 

 miles farther south-west, in the northeastern 

 corner of Arizona, lie the Carriso Mountains, 

 the laccolitic nature of which is not positively 

 stated. El Late Mountains, in the southwestern 

 corner of Colorado, are believed to be laccolites. 

 Next, the Abajo Mountains of eastern LTtah are 

 compared with the laccolites of the type area, 

 and the La Sal Mountains, about 35 miles north 

 of the Abajo Mountains, are only doubtfully 

 considered as due to intrusions. In discussing 

 the conditions of intrusion in laccolites, Mr. 

 Cross concludes in agreement with Dana that 

 Gilbert's explanation of the incoming of the 

 magma into the strata is complete without re- 

 ference to the relations which may exist be- 

 tween the densities of the lava and the strati- 

 fied rocks. 



Comment: The time of formation of lacco- 

 lites and volcanoes in the same field seems not 

 yet to be fully determined, but in two of the 

 areas described it is probable that laccolites 

 were first formed and subsequently dikes and 

 volcanic rocks were formed, the former appear- 

 ing at the present denuded surface and the lat- 

 ter having poured out upon the surface. If 

 this order should prove generally true, it would 

 agree with that observed in the case of small 

 intrusions in the Boston basin. Thus in the 

 slate area bordering the Mystic River there 

 are at least three series of intrusions in the form 

 of dikes and sills. The sills, here the ana- 

 logues of laccolites, are in every instance con- 

 nected with the earliest movements of the, 

 magma. Moreover, the sills came in before the 



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