ITALIAN PETROLOGICAL SKETCHES 35 I 



anorthite — along with orthoclase, and such rocks will be called 

 collectively in this paper by the name of trachydolerite, a name 

 proposed by Abich' as far back as 1841. This term seems the 

 more appropriate since one of his type rocks is that of Monte 

 Santa Croce at Rocca Monfina, which has the mineralogical 

 characters of one of the subgroups with the chemical composi- 

 tion of another. For those intermediate effusiv^e rocks in which 

 the plagioclase occurring along with orthoclase is acid — ande- 

 sine to oligoclase — the name trachya?idesitc,'^ which is in use in 

 France, will be reserved. 



Cimiidtc. — The rocks belonging to this group are porphyritic 

 in structure, the phenocrysts being of augite, olivine, and some- 

 times feldspar, and the rather light to dark groundmass being 

 seen under the microscope to be generally a hoiocrystalline 

 paste of feldspar with augite and magnetite grains, glass base 

 being rare. The lamprophyric habit of some of the specimens 

 has been remarked on. The ciminites are characterized mineral- 

 ogically by the presence of alkali feldspar and a basic plagioclase, 

 augite and olivine, with accessory mag^ietite and apatite. Biotite 

 and hornblende (especially the latter) are either entirely absent 

 or only present in small amount. Chemically they are rather 

 basic, silica varying from 54 to 57 per cent.; alumina is mode- 

 rately high, as are the iron oxides ; magnesia and lime quite 

 high, respectively 3 to 6 and 5 to 9 per cent. Alkalis are low 

 for the rocks of the region, but potash is uniformly higher than 

 soda. These rocks do not seem to be very abundant, except at 

 the Monti Cimini, where they apparently occur in large quan- 

 tities. 



In Table I are given all the reliable analyses of these Italian 

 rocks which are known to the writer, together with those of the 

 rocks of Radicofani. Of these No. i of the flow at Fontana di 

 Fiesole, near Viterbo, may be regarded as typical. Vom Rath's 

 analysis (No. 2) of a similar rock from the same region closely 

 agrees with this, the chief discrepancy being found in the alkalies, 



' Abich, Vulk. Erscheinungen, Brunswick, 1S41, 100. 



^This term has been used in a more general sense in the preceding papers. 



