AMERICAN GEOLOGY DECADE OF 1850-1859. 4o3 



that while the greater part of the work was accomplished before the 

 continent hud emerged very considerably from the water, nevertheless 

 the work of erosion went on for some time after emergence began. 



In his discussion relative to traces of ancient glaciers he called atten- 

 tion to the occurrence of two series of stritv. differing- in their direc- 

 tion, the more extensive being what he termed " drift stria'," and the 

 second, much more limited in extent, "glacial strise," the latter being 

 confined to the valleys. The first he thought due to aqueo-glacial 

 causes; the last to glaciers existing earlier than the drift. Further 

 investigation brought to light other stria 1 which seemed quite as recent 

 as any caused by the drift agency, and he confessed to a feeling of 

 doubt as to which of the agencies was earliest. " Perhaps there were 

 two periods of glaciers, one before and one subsequent to the drift. 



The facts concerning the dispersion of bowlders he thought could be 

 more satisfactorily explained by icebergs than by glaciers, since this 

 transportation and scattering continued till after the time when a large 

 part of the beaches and terraces were formed. Glaciers would have 

 plowed tracks through stratified deposits. Icebergs such as now 

 traverse the Atlantic might carry bowlders over the beaches and ter- 

 races and drop them from time to time, forming thus the intermixture 

 of coarse, angular blocks and beach or terrace material as we now find 

 it. "The supposition that a glacier once existed on this continent, 

 wide enough to reach from Newfoundland to the Rocky Mountains, is 

 the grand difficulty in the way of the glacial theory." (See further 

 on p. -171.) 



The illustrations accompanying this paper were beautifully exe- 

 cuted, perhaps the best, both those in color and in black and white, 

 that had thus far been produced. 



A second work of Hitchcock, published this same year and needing 

 a brief reference, was his Geology of the Globe, a small octavo volume 

 of 136 pages, 6 plates of fossils, and 2 colored geological maps, one 

 of the United States and Canada and one of the world. 



The source of inspiration was evidently Boue's treatise published 

 under the auspices of the Geological Societ} r of France. On the maps 

 the various formations, indicated by color, were: (1) Hypozoic and 

 metamorphic strata, with granite, syenite, and some porphyries; (2) 

 Primary fossiliferous strata to the top of the Carboniferous; (3) Sec- 

 ondary strata; (4) Tertiary strata; (5) Alluvial deposits; and (6) Vol- 

 canoes and igneous rocks of the Alluvial and Tertiary periods. 

 Comparatively few areas are left uncolored — areas scarcely larger than 

 those left as unknown or uncertain on the map published in 1884 by 

 the official survey of the United States. When one thinks of this he 

 can but smile at the naive remark of the author when referring to the 

 coloring: "Nor can I doubt that it is done approximately correct." 



