THE ERUPTIVES. 353 



abundance of a basic mineral — that the most acid feld.sj)ar is found. These 

 determinations are from the fresher rocks. Usually the feldspar is con- 

 siderably decomposed, while not infrequently some . unaltered jdagioclase 

 remains. The must usual alteratit)n of the feldspars has resulted in giving 

 it a gray turbid appearance, which is taken to be a kaoliuitic decomposi- 

 tion. Frequently the inclosed flakes are lai'ge enough to give brilliant 

 polarization colors. Next, to this kaoliuitic decomposition in frequency is a 

 change to green pleochroic needles, which are amphibole, probable variety 

 smaragdite. Independent needles of this kind not only occur in great 

 abinidance throughout the feldspar, but others have penetrated it, coming 

 from the enlargement of augite and paramorphic hornblende. In some 

 cases the feldspar is penetrated in every direction by these needles, 

 so that but little of the original material remains. At times a green chlo- 

 ritic substance is contained within the feldspars, within which is developed 

 epidote. Not infrequently, also, small blades of secondary biotite are 

 found included. 



The pyroxene is generally of the augitic variety, although in the 

 rocks .which apjiroach gabbros it takes on a diallage parting. It has a 

 slight pleocln-oism, varying from colorless, to very pale reddish brown. In 

 no section is tlie pyroxene entirely unaltered, while in many sections there 

 only remain here and there detached cores of the original mineral, and 

 not infrequently it lias entirely decomposed. The most common alteration 

 is to ordinary hornblende. The angle c -.Q of the secondary hornblende is 

 rather high, being in some cases more than 20°. Its pleochroism is normal. 

 Ordinarily C is gi'eenish blue, b liluish gi'een, and a })ale bluish green to al- 

 most colorless. Less frecjuently C is yellowish green, b greenish yellow, and 

 a a light yeUow. Rarely tl,ic alteration is to tyi)ical l)asaltic hornblende. 

 Here C is dark brown, b brown, and a a light yellow. The absorption is C ^ b 

 > a. The; liornblende is usually paramorphic, a single large individual of 

 hornblende resulting from one of pyroxene. The rare diallagic variety of 

 pyroxene has geneivdly altered to a green fibrous amphibole, which is taken 

 to be smaratrdite. Hut tlie most interesting fact in connection with the 

 change of augite to amphibole is that the areas now found are at times 

 •larger than were the original augites; that is, a growth subsequent to the 



MON XIX -'3 



