THE NATURAL HISTORY OF IGNEOUS ROCKS. 29 



and shonkinite of Weed and Pirsson, besides dykes of 

 solvsbergite (the acmite-trachyte of Wolff and Tarr), Iddings' 

 absarokite and allied types. Apart from the significance of 

 the laccolitic habit, of which something may be said in a 

 future number, the petrographical uniformity of the in- 

 trusions in the former case seems to connect itself with the 

 geotectonic structure of the country in which they occur. If 

 the physical process, whatever their nature, which give rise 

 to diversity of intrusive rock-types stand related in some 

 degree to mountain-building forces of the horizontal kind, 

 a smaller range of varieties is to be expected in association 

 with the monoclinal folds, normal faults, and other evidences 

 of vertical movement which characterise what American 

 geologists have styled the Great Basin type of mountain- 

 structure. If this be so, we should look for a greater 

 variety of intruded rocks in the true mountain tracts farther 

 west. Concerning Tertiary intrusions in the Sierra Nevada 

 there seems to be little information, but in the Coast 

 Ranges of California the intrusive rocks presumably of this 

 age include diabase, gabbro, peridotite, pyroxenite, and 

 other types. 



If highly specialised rock-types and very local distribu- 

 tion of the different types may be taken as the index of 

 advanced evolution in a petrographical province, the 

 Tyrrhenian province has arrived at a high stage of develop- 

 ment. The assemblage of rock-types there belongs, as 

 already remarked, to the alkali group. It is true that 

 andesites occur among the earlier volcanic products of the 

 yEolian Islands and again in the Cabo de Gata district, though 

 some of the latter have unusual characters. The later 

 lavas, however, throughout the province, cover a wide range 

 of types, which for the most part carry a high content of 

 alkalies. Some, such as the remarkable acid lavas of Pan- 

 tellaria and the leucite-bearing rocks of Vesuvius, are unique 

 and of strictly limited occurrence. Among active or recently 

 active volcanoes we find some in close proximity to one 

 another producing widely different lavas. Thus in the 

 Lipari group the basalts of Stromboli are contrasted with 

 the rhyolites of Vulcano, etc., and with the rocks of the 



