FEy^ER, THE WATCHVNG BASALT 16] 



birefringence. In the second period, the characteristic minerals are soft, 

 the refraction is less than that of balsam and the birefringence is very 

 low. 



In the glassy crusts of basalt, there is frequently a lamellated appear- 

 ance, which is doubtless an original structure developed at the time of 

 consolidation. In addition to the parallel parting-surfaces due to this 

 feature, the glass is cut by numerous chill cracks running in every direc- 

 tion, so that only a little force is required to break it into a multitude of 

 angular fragments. Definite openings of these kinds, though of capil- 

 lary dimensions, doubtless offered the means for the first ingress of 

 waters, and along these cracks, alteration first attacked the rock. This 

 is well shown in slide 30, illustrated in Plate XII, fig. 2. Along the 

 cracks, which show a predominant parallelism, the glass has become 

 bleached to a much lighter color and shows a hazy birefringence. The 

 lighter portions represent the first step in alteration and probably consist 

 of an intermixture of zeolites and chlorite, so complete that it is entirely 

 beyond the powers of the microscope, and no individual crystals can be 

 perceived. It is worthy of note that among the first effects there is 

 always a distinct bleaching, indicating removal of ferromagnesian com- 

 pounds. 



In addition to the openings by which the solutions were directed, there 

 were present in the glass a number of features of physical or chemical 

 nature which strongly influenced the course which alteration first took 

 and traces of which remained at a stage at which recrystallization had 

 become complete. Such were the phenocrysts of plagioclase and diopside, 

 bubble cavities, and, perhaps more marked in their effect than all others, 

 the nodules of olivine which were in process of resorption when solidifica- 

 tion occurred. In the refusion of olivine, each nodule supplied a source 

 of material somewhat different in composition from the average of the 

 magma, and this material streamed out along flow-lines before being in- 

 corporated in the liquid. The interruption of the process by stiffening 

 and solidification fixed and retained these differences of chemical com- 

 position. Effects of this kind may not be perceptible in the glass, but 

 they are accentuated in the process of alteration. Brecciation of the 

 glass and an overwhelming of the fragments in a fresh supply of lava 

 also probably had its effect in determining differences of a physical or 

 chemical nature and gave rise to a lack of homogeneity, which was ac- 

 centuated by subsequent alteration. Various features of this kind influ- 

 enced the course of alteration, and their results may be traced in many 

 of the slides. Phenomena which are ascribed to them are shown in 

 Plate XII, figs. 2, 3, 4 and 6, and Plate XIII, fig. 3. In some cases, the 



