ENDucH.] ERUPTED KOCKS CLASSIFICATION. 203 



groups are the most important. In France the mountains of the Au- 

 vergne region are composed of trachyte; in Central Germany the 

 Drachenfels group, formed by the same rock, has become famous; and 

 the basalts along the Rhine and some of its tributaries are well known. 

 Toward the eastern portion of Germany, trachytic series again occur, 

 denoting their presence by the formation of isolated hills or mount- 

 ain groups. The "Vogelgebirge" of Hessia and the isolated outcrops 

 throughout Southern Germany belong to the Basaltic era. To the lat- 

 ter, also, we count the volcanics of the British islands and of Sweden 

 and Norway. 



Showing closer affinities, perhaps, than the volcanic groups of any 

 other region, with those of Colorado and the Eocky Mountains gener- 

 ally, are those of Transylvania. Kichthofen, whose study of our west- 

 ern country has been productive of such excellent results, has given his 

 attention to the volcanics of that country, and to him, besides in a great 

 measure to Beudant, we owe the knowledge of an existing similarity 

 between the two. 



In a general way, it is possible to correlate the volcanics of the two 

 continents. In North America, however, we find all the features repre- 

 sented on so grand a scale that direct comparison will furnish but un- 

 satisfactory results. In variety of forms produced, and variations of 

 constitutions exhibited by volcanics, our North American are unique, 

 though not entirely distinct from the European. We deal with broader 

 areas, with phenomena produced on a very large scale, and the differ- 

 ence, first, between foreign occurrences, and, secondly, among the North 

 American volcanics themselves, is proportionate thereto. 



From the comparatively incomplete observations that are at our 

 command at the present time, respecting the volcanic formations of the 

 western portion of our continent, we are enabled to say that our series, 

 speaking from an evolutionary stand poiut, is by far more complete than 

 the European. It is in the West but a step from the prehistoric to the 

 active volcanoes, while east of the Atlantic the difference is strongly 

 marked. While we are often in doubt whether an occurrence of scoria 

 or lava may be even more than a few hundred years old, the former 

 volcanoes in the interior of Europe present a more ancient type, remind- 

 ing less of the characteristic material that composes and issues from 

 volcanoes of to-day. It is this comparative completeness of our series 

 that produces the difficulty in devising any thoroughly satisfactory sys- 

 tem, which will not only define the numerous lithological varieties, but 

 at the same time bear distinct and definable relations both to genesis 

 and age of the rocks thus classified. 



As proposed by Richthofen,* the classification for the volcanic rocks 

 of our western mountain-ranges is one based essentially upon their age. 

 Within certaiu limits they all belong to a comparatively restricted geo- 

 logical period, and by determining their relative positions, very appli- 

 cable data can be obtained. Both in EuropiB and America certain groups 

 have been recognized as bearing definite relations to each other, and the 

 determinations made on both continents agree very well. For all prac- 

 tical purposes the system as introduced by Richthofen certainly an- 

 swers, and, as it is one designating minor divisions, it can be utilized in 

 conjunction with any more or less artificial classification that furnishes 

 the characteristics for primary groups. In the subjoined pages, there- 

 fore, this classification shall be adopted in the main, with such addi- 

 tions as may seem to be required, and in connection with a primary 

 division based upon simple chemical constitutions. 



* Mem. Cal. Ac. Sci, 1868. Vol. i, part ii. 



