LINOSA AND ITS ROCKS 19 



camptonose, and II camptonose-auvergnose. Collectively, therefore, 

 these rocks may be considered to belong to the transitional divi- 

 sion camptonose-auvergnose, as they apparently tend to be 

 docalcic. 



Relations of norm and mode. — In these rocks the relations of norm 

 and mode are quite simple and the readjustments needed to compute 

 the latter from the former are confined almost wholly to those neces- 

 sary to form the modal augite. As this mineral has not yet been 

 analyzed, it is needless to go into any great detail, but it may be 

 pointed out that these readjustments would, on the analogy of many 

 other similar cases which have been worked out, consist in the transfer 

 of a little of the normative anorthite, magnetite, and ilmenite, com- 

 bined with the whole of the normative diopside, to the more complex 

 modal augite molecule. The normative orthoclase, but little of which 

 is present, must be considered as entering the modal plagioclase, though 

 a small part of it may be present in the base of the ground-mass. As 

 the augite is not markedly titaniferous, and the ohvine presumably 

 not more so, and as no titanium minerals, as perofskite, titanite, or 

 ilmenite,. are present, the very considerable amount of normative ilmen- 

 ite (except that which enters the augite) must be held in the opaque 

 ore grains, which, therefore, are to be considered as titaniferous 

 magnetite. 



But all of these readjustments are of comparatively small magni- 

 tude, even those needed for the formation of augite, which are more- 

 over the normal ones, or those necessary in the case of the vast majority 

 of igneous rocks, in which the pyroxene is aluminous. The modes 

 are, therefore, normative and, in view of the abundance of feldspar 

 phenocrysts, the Monte Ponente type may be briefly described as 

 salphyro-camptonose-auvergnose, which indicates very concisely a 

 basalt of about the chemical composition shown by the analyses, 

 showing the mode in general described above, and with prominent 

 feldspar phenocrysts. 



The presence of a slight amount of quartz in the norm of II may 

 seem to be inconsistent with the presence of olivine in the mode. But 

 this is by no means uncommon, and instances of basalts containing 

 ohvine in the mode, while the norm shows none and even some quartz, 

 are met with quite frequently, as along the Pacific slope of the United 



