326 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The feldspar is substantially a mixture of soda and potash molecules. 

 In the phenoerysts of the porphyrias much of it appears as a homo- 

 geneous mixture of the two. Irregularly through parts of the pheno- 

 erysts, and toward their margins, cryptoperthite or microperthite 

 appears. A later growtli of fine microperthite attached to the pheno- 

 erysts, but of groundmass age, occurs. The feldspar of the ground- 

 mass is microperthite. In the granites, the feldspar is substantially 

 all microperthite, a little free albite occurring about the ends and sides 

 of the crystals. The microperthite throughout, so far as can be told, 

 is a microcline-albite microperthite. Later changes of a deep-seated 

 character have profoundly altered much of the phenocrystalline 

 feldspar of the porphyries, either recrystallizing it to an aggregate 

 of albite and microcline, or in part replacing it with albite. Similar 

 replacements have occurred to some extent in the granitic feldspar. 



The predominant dark silicate is hornblende, of which two varieties 

 appear. One of these is riebeckite or a closely allied type, the other 

 appears to be closely related to the cataphorites. Both are generally 

 present: on the whole the riebeckitic Aariety clearly predominates 

 and hence the rocks have been characterized as riebeckitic. The 

 predominant pyroxene, by a wide margin, is aegirite. In the most 

 basic member of the rock-series a variety of pyroxene rich in the cal- 

 cium-ferrous iron molecule appears. This is found to a very small 

 extent in the more siliceous porphyries but gives place to a more 

 highly sodic type, which also occurs in the granites to a small extent. 

 The rare mineral aenigmatite occurs in the granite and in the granite- 

 porphyry. 



The predominant molecules in the hornblendes are : — Na2Fe2Si40i2, 

 (R'ojR") Fe2Si40i2^ — where R' is soda with small amounts of fluorine 

 and hydroxyl, and R", ferrous iron chiefly — and Fe4Si40i2: In the 

 pyroxenes Na2Fe2Si40i2 strongly predominates but Ca Fe Si206 is 

 often present. The conditions determining the formation of these 

 minerals is very obscure, but slight variations in the concentrations 

 of the constituent oxides present, local variations in the amounts of 

 mineralizers, particularly fluorine, and the rate of cooling, are doubtless 

 the most important factors concerned. 



The hornblendes are readily effected by alteration, and change first 

 to a deep-blue, riebeckitic or crocidolitic variety and eventually 

 change to magnetite and other iron oxides. The aegirite is less readily 

 altered. 



Alteration, deep-seated or superficial has not much effected the 

 granites, except locally, but the porphyritic rocks have suffered more 



