20 HENRY S. WASHINGTON 



States,^ and in the fiescolal ciminose (ciminite) near Viterbo, wHich 

 has been discussed elsewhere.^ In such cases the analysis leaves no 

 doubt that there is an excess of sihca which, owing to the modal pres- 

 ence of olivine, must be increased in the mode over that shown by the 

 norm, and is to be looked for either in the residual base of the ground- 

 mass or, as in the quartz-basalts, is present as clearly recognizable 

 quartz. 



Occurrence. — Basalts of this type are especially well represented 

 at Monte Ponente, composing the extensive northern flows of this 

 cone as well as the dike which cuts the tuff of Monte Pozzolana. 

 Other representatives of the type are met with in many of the flows of 

 Monte Vulcano, some of those of Monte Rosso and Monte Raneri, 

 and in the basalts about the Scala Vecchia, which are probably derived 

 from Monte Vulcano. 



Gypsum in vesicles. — In parts of one of the lower flows of Monte 

 Ponente, as noted above, the vesicles of the lava contain a white, 

 crystalline substance, which was supposed to be a zeolite, in the field. 

 Examination in the laboratory, however, shows that it is gypsum, 

 and as such an occurrence is unusual for this mineral it merits a brief 

 description. 



The mineral forms thin crusts, which are distinctly crystalline, 

 though no definite crystal forms are to be seen. It is soft (H = about 

 2), and slight crushing develops poorly defined cleavage flakes. Under 

 the microscope these show rather low refraction and double refrac- 

 tion, which were not determined quantitatively. Two other cleavages 

 are prominent (one better developed than the other), crossing at about 

 114°, that of the two minor cleavages of gypsum. The acute bisec- 

 trix Hes in the acute angle of the cleavage rhomb, making angles with 

 the cleavage trace parallel to loi varying from 12° 30' to 14°, the 

 corresponding angle for gypsum being 13° 39'. An analysis (on 

 o. 2122^"^) gave the following results, the calculated values for gypsum 

 being given for comparison: 



1 See, for instance, Professional Paper, U. S. Geological Survey, No. 14, 1903, 

 p. 289, hessose, Nos. 18, 23, 29, and p. 333, auvergnose, No. 32. Many other cases 

 could be cited from this collection. 



2 H. S. Washington, The Roman Comagmatic Region, Carnegie Publication 

 No. 57, 1906, p. 64 



