/ TALI A N PE TR OL O GICA L SKE TCHES 835 



to those described | trachytes]." We have already seen that his 

 "trachyte" from Casaccia probably belongs here. Mercalli also 

 calls the Cimino rock an oHvine-trachyte. 



I was unfortunately unable to obtain specimens from the 

 main mass of Monte Cimino, but those which I collected from the 

 flows back of Madonna della Querela and at Fontana di Fiesole ^ 

 are of essentially the same type of rock. A specimen from below 

 San Rocco in the Vico crater probably belongs to this latter 

 center of eruption. 



These rocks are all very compact, one of my specimens from 

 Fontana di Fiesole alone showing a few elongated vesicles. 

 Their color is light gray, that from the Vico crater being slightly 

 greenish. In the fine-grained groundmass are scattered abun- 

 dant small phenocrysts of pale yellow olivine, with small black 

 augites, and fewer small glassy sanidines, the rock from Madonna 

 della Querela showing the most of these last. The Fiesole rock 

 has a sp. gr. of 2.70 at 10'^ C. 



Under the microscope they present much the same features, 

 the only marked differences being in the greater or less develop- 

 ment of feldspar and pyroxene in the groundmass. Feldspar 

 phenocrysts are very rare in the slides and are seen to be both 

 of orthoclase and plagioclase. No sections of the latter were 

 found by which its exact character could be definitely deter- 

 mined, thouorh it seems to be a rather basic labradorite. The 

 feldspar phenocrysts are much corroded. 



There is a great abundance of large and small phenocrysts of 

 olivine, but this does not occur as a true groundmass constituent. 

 They show the normal forms, more or less corroded, and are 

 colorless and perfectly fresh in the interior. All show, however, 

 a narrow, bright reddish-brown border, so frequent on olivine, 

 and apparently of the same substance as that which has received 

 the name of iddingsite.- This covers original crystalline, cor- 

 rosion, and fracture surfaces alike, though here and there a small 



' Dekcke (op. cit., 240) states that this flow probably belongs to the Bolsena cen- 

 ter, lie does not give the reasons for this view, and my own observations leave no 

 (.loubt in my mind that it belongs to Monte Cimino. 



= A. C. Lawson, Bull. Dept. Geol., Univ. Cal., I, 31, 1893. 



