304 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



country, the exact provenance of which is unknown. Thus may 

 petrology aid archaeology. 



The long chain of the Andean volcanoes seems to form a continua- 

 tion of the main Cordilleran region, which is continued northward 

 along the Aleutian volcanoes, and thence southward, along the west 

 coast of the Pacific, through the volcanoes of Kamchatka, Japan, 

 and the Philippines, and so on to the Dutch East Indies. This so- 

 called " Circle of Fire " surrounds a large area, that of the Pacific 

 Islands, whose rocks are dominantly basaltic — that is, low in silica 

 and alkalies, and high in lime, magnesia, and iron, associated here 

 and there with occurrences of alkalic rocks. 



In Europe the various comagmatic regions are so numerous, so 

 complex, and so little known from this point of view, that only a few 

 need be mentioned. There is the extensive, though broken-up, region 

 that embraces the British Islands and their outliers, with Iceland, 

 eastern Greenland, Jan Maj^en, Spitsbergen, and Nova Zembla, the 

 rocks of which are dominantly basaltic. The highly sodic Chris- 

 tiania region in southern Norway has been well studied by Brogger, 

 as has the calcic Bergen region by Vogt. Germany and Austria are 

 filled with a complex of different regions, the relations of which are 

 not yet clear, but which seem to be either dominantly sodic, as that 

 of Bohemia, or with basaltic tendencies. The Alps and the Tyrol 

 form a central region of prevailingly granitic rocks which differ 

 markedly from the various and different regions that surround them ; 

 this is a point to which we shall recur. In Italy is the so-called 

 Roman comagmatic region, embracing the volcanoes along the west 

 coast from Bolsena to Vesuvius, the rocks of which are decidedly 

 unusual in their very high potash, with considerable lime. A zone 

 of distinctly sodic rocks appears to extend from southern France and 

 eastern Spain, down Corsica and Sardinia, through the island of 

 Pantelleria, into Tripoli. Hence, by way of Kordofan, this region 

 is possibly connected with the highly sodic one that stretches from 

 Abyssinia down the Ethiopian Rift Valley, in East Africa, and 

 which branches northwardly along the Red Sea and Arabia as far as 

 Syria. At the east end of the Mediterranean, on the other hand, is a 

 region embracing Greece and the Balkan Peninsula, the Archipelago, 

 and western Asia Minor, whose rocks resemble very closely those of 

 the Colorado plateau and of the Andes volcanoes. 



We could go on thus over the surface of the earth, so far as its 

 rocks are sufficiently well known chemically. Unfortunately, this 

 is not the case with many large, and otherwise thoroughly studied, 

 areas or regions, such, as, for instance, the Greater Antilles, Cuba, 

 Jamaica, Haiti, and Porto Rico. But this very rapid sketch will 

 serve to give the reader some idea of how diversified, chemically, are 

 the different portions of the earth's surface. 



