Academy of Sciences] AMERICA 65 



the complex relation of Jurassic ( ?) igneous rocks to one another are well exposed in cliff-sections. 

 There are two main areas of these; the one consists of pillow-lava, showing an extraordinary- 

 assemblage of highly scoriaceous rock, sheet-like masses of elliptical basalt, sometimes sodic and 

 apparently decalcified (see analyses cited and of. Termier '98), together with veins of coarse 

 gabbro of normal calcic composition. The other stratiform intrusive mass shows an extraordi- 

 narily intimate mixture of peridotite and flaser or banded gabbros, in which it is difficult to say 

 which is the older. These are cut by diabase. The descriptions remind one strongly of what 

 may be seen in some parts of the Lizard Peninsula in Cornwall (Flett and Hill '12, pp. 81-101). 

 This region differs from all other parts of the coast in the large amount of gabbro that is present. 



Again to the southeast there are intrusions of serpentine and diabase into supposedly 

 Knoxville rocks in the San Rafael Mountain (Fairbanks '96) , and near San Diego in the south- 

 western corner of California, Lawson ('04) noted an orbicular gabbro doubtless occurring in 

 the same series as the norites, gabbros, diabase, and more acid plutonic rocks which Fair- 

 banks ('93) found to invade the crystalline metamorphic rocks in this region. Further south 

 the succeeding occurrences of such basic rocks in the Islands of Cedros, Santa Magdalena, and 

 Santa Margarita off the end of the peninsula of Lower California are associated with highly 

 altered schists and amphibolites (Lindgren '93) . 



In his interesting summary of the petrographic provinces in North America, Iddings ('13) 

 remarks that the differences between the congenetic series of rocks in different regions are 

 sufficient to distinguish them as representatives of different petrographic provinces, though 

 sometimes they are so slight that there seems to be what Pirsson terms a regional progression 

 of types. "The data already to hand suggest the existence of ill-defined zones traversing the 

 North American Continent along the lines of its chief physiographical features, but they also 

 indicate the petrographic complexity of these zones, and the probability of their being separated 

 into many petrographic provinces and into innumerable petrographic districts." As may be 

 seen from the foregoing summary there is in the relationship of the distribution of petrographic 

 type to that of the tectonic features of the western Cordillera, a very considerable degree of 

 concordance with the generalizations of Dr. Harker. 



CENTRAL AND SOUTH AMERICA. 



Passing farther to the south we note that though there are abundant evidences of immense 

 folding-movements in Mexico, as for example the great recumbent folds of the Cretaceous lime- 

 stones of the Sierra Madre Occidentale, there are no ultrabasic intrusions, so that here, as in 

 much of the Pyrenean and Carpathian Ranges and many other parts of the world, folding has 

 occurred without the injection of ultrabasic rocks; or if these were present, they have not been 

 yet exposed by erosion. The volcanic rocks, which form the bulk of the present tabular ranges 

 have been poured out from fissures formed in the underlying folded range, and no precursors 

 of these eruptions appear in the form of volcanic tuffs embedded in the older sediments (Suess 

 IV, 437-442). 



Suess traces the trend-line of the great Andean folds from the south of Mexico into Guate- 

 mala, where it leaves the southeasterly trend and turns into an east-north-east direction into 

 the Sierra de las Minas, on the north side of which, according to Sapper ('99) runs a band of 

 serpentine, and on the south side another band extends along the River Motagua for 225 kilo- 

 meters. It is regarded by him as younger than the Upper Carboniferous and older than the 

 Middle Cretaceous. Nevertheless, a recent report by Powers ('18) has thrown doubt on the 

 presence of this serpentine, remarking that "the belt mapped as serpentine coincides with 

 a belt of very good quality white crystalline marble which composes the south flank of the 

 mountains." The more metamorphosed rocks, considered to be ancient schists by Sapper (but 

 not by Powers) , extend in a range south of the River Motagua and end in the islands of Ruatau 

 and Bonaca, the latter consisting of serpentine and phyllite. The continuation of this tectonic 

 axis is in Jamaica. Cuba, however, appears as an inserted axis, a virgation of the main line, 

 and in this extended island there is a long zone of serpentine forming the main watershed of the 

 island, though of no great height. It occurs in a complex of metamorphic and igneous rocks, 



