464 



NA TURE 



[March 19, 1896 



chance Prof. Iddings has not been slow to take the fullest 

 advantage. The author does not use the word porphyrite 

 in the sense of an altered or ancient andesite, but applies 

 it to " medium-grained porphyritic rocks that occupy an 

 intermediate position between the coarsely granular 

 diorites and gabbros and the microlitic or glassy ande- 

 sites." In describing the diorites, he gives a plate of 

 •examples of the intergrowth of such minerals as horn- 

 blende, biotite, augite, and hypersthene. The whole of 

 the plutonic rocks are compared together as to structure 

 and chemical composition, and shown to form a connected 

 and overlapping series. Similar comparisons are effected 

 between the volcanic products of Sepulchre Mountain. 

 Mr. Iddings points out that it is impossible to trace any 

 actual transition through stock and dyke rocks into the 

 corresponding eruptive rocks, for the reason that they are 

 often cut asunder while the volcano is m activity, and 

 because the pipe is invaded by each magma in turn, the 

 last alone remaining there to solidify, unless little patches 

 of necessarily altered previous types happen by accident 

 to survive. 



In the nineteenth monograph, Messrs. Irving and Van 

 Hise treat of the Penokee iron-bearing series of Michigan 

 and Wisconsin. This is a monoclinal series of rocks 

 dipping northwards off a mass of crystalline rocks which 

 lie to the south, and extending a distance of eighty miles 

 along their strike. The "Southern Complex" is a mass 

 of schists of eruptive origin, with masses of granite in- 

 truding into them. Resting unconformably on this com- 

 plex, comes a thin series of cherty limestones (300 feet) 

 of aqueous origin, but whether chemical or organic is 

 not known, and unconformably again on that, the quartz- 

 slate (500 feet). The iron-bearing member (800 feet) in 

 its least altered state is a water-deposited cherty iron 

 carbonate, which can be seen to grade into ferruginous 

 slates, these into ferruginous cherts with concretionary 

 and brecciated structures, and these again into actinolitic 

 slates containing much magnetite. This rock is pene- 

 trated by diabase dykes which, when intruded, appear 

 to have been vertical, while the beds were horizontal. 

 It results from this that, now both are tilted, V-shaped 

 areas occur defined by the upper quartzite of the quartz- 

 slate on the one hand, and the dykes on the other. It 

 is in the apices of these V's, that the iron ore is now 

 concentrated, the original rock being rendered very poor 

 in ferruginous constituents, which have been re-deposited 

 in the ore masses as haematite. In the area east of 

 Gogebic Lake, the regular succession is disturbed by 

 volcanic accumulations and greenstone conglomerates. 

 The Upper Slate member (12,000 feet) rests on the iron- 

 bearing rocks, but it does not extend quite so far as they 

 do. The southern complex is placed in the Archaean 

 system, the overlying beds into the Algonkian ; these 

 are covered by the Keweenawan series, after which the 

 rocks received their monoclinal tilting ; finally, the 

 Eastern Sandstone was unconformably deposited on 

 them all. The work is illustrated by a large series of 

 illustrations of the microscopic structure of igneous 

 and sedimentary rocks, and by maps and sections. 



Mr. Walcott * gives an admirably illustrated account of 

 the igneous rocks of the Unkar terrane, a group which 

 underlies the Tonto Sandstone in the district of the 

 Grand Canon of the Colorado, and has been variously 

 referred to the Algonkian, the Cambrian, and the Silurian 

 by himself and other authors. The presence of a well- 

 marked Middle Cambrian fauna in the upper part, and 

 a strong unconformity at the base of the Tonto Sand- 

 stone, are sufficient in the opinion of the author to 

 warrant our considering the Unkar beds as Pre-Cambrian, 

 and correlating them with part of the Algonkian suc- 

 cession. The upper part of the Chuar terrane, which 

 immediately unoerlies the Cambrian rocks, consists of 



1 Fourteenth Annual Report of the Geological Survey of the United 

 States, 1892-93. (1894.) 



NO. 1377, VOL. 53] 



1700 feet of shales and sandstone, with 138 feet of lime- 

 stone ; the lower division of this terrane has 3420 feet of 

 similar rocks, with 147 feet of limestone of a rather 

 different type. Two of the limestones are spoken of as 

 Stromatopora limestones, and they contain an organism 

 which is probably a species of Cryptozoon. 



The Unkar terrane which underlies that last de- 

 scribed, is 6830 feet thick ; there are magnesian lime- 

 stones at the top, followed by lava beds 800 to 1000 feet 

 thick, then sandstones and another lava bed from 80 to 

 180 feet thick on a limestone and conglomerate, which 

 rest unconformably on the Vishnu schist, and the gneisses 

 and schists of the Archaean System. The igneous rocks 

 were probably submarine flows poured out from fissure 

 eruptions, the dykes of which are still visible in the walls 

 of the caiion, during a slow subsidence when beds of 

 sandstone and shale were interleaved with the volcanic 

 products. 



Mr. Iddings, who contributes an appendix, describes 

 the lavas as olivine-basalts sometimes fresh but more 

 usually altered, the felspar passing into saussurite, the 

 pyroxene to chlorite, and the olivine to serpentine and 

 other products. When the original structure is well 

 preserved, it does not differ much from that of the 

 Tertiary lavas of the same area. The ground-mass of 

 the basalts is usually microcrystalline, but in the middle 

 of thick sheets it becomes ophitic, while the upper and 

 under parts are scoriaceous and amygdaloidal. 



In that lucid style for which his work is well known, 

 Mr. Walcott^ sketches out the physical history of the 

 North American continent during Cambrian time. He 

 shows that Dana's idea of the existence of a V-shapcd 

 skeleton, about which Palaeozoic sediments accreted, is in 

 the main correct, but he is able to fill in a number of 

 details which were of necessity left blank by that author. 

 The underlying massif is composed of Algonkian rocks 

 resting unconformably on those of Archican age ; its 

 rocks were tilted and uplifted into a land area, which 

 Walcott proposes to call the Algonkian continent, and 

 which was of wider extent than any land in this position 

 until Mesozoic time. This continent consisted of four 

 chief parts : (i) the V-shaped mass running parallel to 

 the outlines of Hudson Bay, and possibly extending in a 

 shield-like area to Texas and the Colorado River ; (2) a 

 PaUeo-Appalachian range, with sub-parallel chains and 

 spurs; (3) a Palaso- Rocky Mountain mass; and (4) a 

 Palaso-Sierra Nevada mass. Sedimentation took place in 

 all the seas defined by these land areas in early Cambrian 

 (O/^w^/Zi^i-) times; that is to say, in (i) the Atlantic Coast 

 Province east of the Palaeo-Appalachians ; (2) the narrow 

 sea extending from Labrador to Alabama ; and (3) on 

 the west side of the great V-shaped mass. 



In Middle Cambrian times there was little change in 

 the areas of deposition, except that partial barriers 

 erected along the line of the Appalachians only allowed 

 a small part of the typical {Paradoxides) fauna to pene- 

 trate westwards from the Atlantic basin. Strongly defined 

 zoological provinces evidently existed in Middle Cambrian 

 time, as proved by the study of the fauna of this period 

 to the west. 



Steadfast depression then began, and the Upper 

 Cambrian Sea carried its sediments over the whole of 

 the great south central region, including almost the whole 

 of the United States as far north as Chicago, and from 

 the Sierra Nevada to the Appalachians. The Appala- 

 chian barrier was, however, strengthened so that the 

 fauna of the Atlantic coast in Upper Cambrian times was 

 related to that of Europe, and quite distinct from that 

 of the Central States. This great depression brought on 

 the limestone deposits of the Ordovician systern. The 

 memoir is illustrated by several important geological and 

 palaeo-geographical maps. 



1 Twelfth Annual Report of the Geological Survey of the United States, 

 1890-91. (1891.) 



