14 HENRY S. WASHINGTON 



the various cones, each of which may be easily identified, as well as 

 the general appearance of the island. 



PETROGRAPHY OF LINOSA 



The lavas of Linosa are all feldspar-basalts according to the 

 prevailing rock classifications, though a few contain very small 

 amounts of nephelite, but scarcely sufficient to entitle them to the 

 name of nepheHte-basalt if account be taken of the quantitative 

 relations. According to the quantitative system of classification 

 recently proposed,^ they are similarly monotonous, all of those 

 analyzed and presumably all the others collected, so far as can be 

 judged from the microscopic examination, falling in the subrangs 

 camptonose (III. 5. 3. 4) and auvergnose (III. 5. 4. 4-5). Miner- 

 alogically they are very uniform and simple, the only constituent 

 minerals being plagioclase, augite, olivine, and titaniferous magnetite, 

 with occasionally some glass and traces of nephelite in a few instances. 

 In the basalts themselves neither hornblende nor biotite could be 

 detected, though crystals of hornblende occur loose in scoria at one 

 or two points, as was noted above. The tuffs have apparently been 

 derived from basaltic magmas, and will be briefly described after the 

 basalts. 



While the basalts vary from quite compact to highly vesicular and 

 scoriaceous forms, such differences may be disregarded as being 

 purely adventitious. They are all porphyritic, phenocrysts of olivine 

 and of augite being present in every case, accompanied by abundant 

 ones of feldspar in many of the flows, while in others feldspar pheno- 

 crysts are rare. But in the great majority of cases these megascopic 

 differences largely disappear in the thin section, and even megascopi- 

 cally the extremes grade into one another to such an extent, and are 

 connected by so many transition forms, that, as regards the majority 

 of the rocks, a well-marked separation of the two is impossible, while 

 the extremes may be referred to two distinct types, chiefly characterized 

 by the abundant presence or almost complete absence of feldspar 

 phenocrysts, as well as by certain slight chemical differences. A third 

 type is more distinct, but even this would probably be found to inter- 

 grade with the others, were more specimens available. 



I Cross, Iddings, Pirsson, and Washington, Quantitative Classification of Igneous 

 Rocks, Chicago, 1903. 



