FENNER, THE WATCHUNG BA8ALT 139 



eye to be wholly stilbite, but with the microscope it is found that this is 

 replacing albite, of which numerous remnants are seen. Similarly in 137, 

 small albite crystals are being replaced by chabazite. In 123, veinlets, 

 which may be due either to shrinkage on cooling or to shear movements, 

 contain albite, datolite and calcite, whose succession is in the order given. 

 In 129, a large crystal of apophyllite carries several isolated inclusions of 

 turbid-looking and much corroded albite of the larger type described. 



Numerous other slides show more or less albite, in some places in 

 rather fresh-looking crystals and in others in isolated groups of greatly 

 clouded appearance. Occasionally its alteration has proceeded so far that 

 the only trace of its former existence lies in turbid patches in other 

 minerals, suggestive of albite merely by their form. 



As to the mode of occurrence of albite in these rocks, it has been found 

 most frequently in minute veins, but this may be due to the fact that 

 under such circumstances the formation of the first products of alteration 

 would tend to seal up the veins and render it difficult for later solutions 

 to enter and attack the albite. In 95, the relations are different. Small, 

 amygdaloidal cavities are filled with secondary products, among which 

 albite and green amphibole and minute grains of garnet appear. It seems 

 also that phenocrysts of diopside of the primary crystallization of the 

 magma have been replaced by albite groups, illustrating two facts observ- 

 able in many cases in these rocks, namely : that ferromagnesium minerals 

 tend to disappear and that the replacing minerals often pay little atten- 

 tion to the composition of the material replaced. Slide 62 also shows this 

 replacement of diopside by albite. Slide 100 illustrates again the filling 

 of amygdaloidal cavities by albite and amphibole, but in this instance 

 later stages of alteration are shown, and chabazite, calcite and chlorite are 

 prominent. 



In several slides, albite is found in a mode of occurrence which has 

 been referred to before in discussing the manner in which solutions at- 

 tacked the rock. The original texture of the normally crystalline basalt 

 is preserved, but the mineralogic make-up is quite altered. In such cases, 

 the laths of labradorite of the original have frequently been altered to 

 albite. Slides 62 and 118 show this effect plainly. 



The position of albite in the replacement series is determined by the 

 minerals in respect to which it has been found to exhibit instability. 

 Datolite, prehnite, pectolite, chabazite, stilbite, natrolite, apophyllite and 

 calcite have all been found in relations which indicate energetic corro- 

 sion and replacement of albite, and these minerals are therefore regarded 

 as belonging to later periods of alteration. This conclusion is confirmed 

 by the relations of these minerals among themselves and to other min- 



