158 MINERALOGY AND LITHOLOGY. 



ground mass is without crystalline form. The rock contains many 

 microscopic cavities, which are filled with a quite complex mixture of de- 

 composition products. The outer walls of the cavities were first lined 

 with sphaerosiderite, then there was a growth of hexagonal calcite crys- 

 tals, and finally the cavities were filled with analcite, the peculiarities of 

 which have been described in the mineralogical chapter. 



Another specimen of this rock, also from Campton falls, offers some 

 other interesting microscopic peculiarities. The external appearance of 

 the rock is the same, but decomposition has produced substances of 

 different aspect. The chlorite in the ground mass is replaced by a dull 

 white translucent substance, which is probably carbonate of lime, and 

 the cavities filled with minerals are absent. The large crystals are con- 

 centrically banded, the different zones resulting from some differing 

 conditions at stages of their growth, and from subsequent alteration. 

 The plagioclase crystals have impurities heaped in their centres, while 

 on the outside the crystals are clear. The augite is in zones, which 

 differ but slightly in their color, but which are brought into stronger 

 contrast when polarized light is employed ; for when the Nicols are 

 crossed and the section is revolved on the stage of the microscope, the 

 different bands do not become black at the same time, which shows that 

 the planes of elasticity in the different parts of the crystal take slightly 

 different directions, and therefore, whatever be the position of the Nicol 

 prisms, the crystal sections are banded with different colors. These 

 augite crystals are represented in Fig. 3 on PI. 9. The crystals of olivine 

 are also quite peculiar in their mode of decomposition. They are 

 affected to their centres, yet the cores have still, in polarized light, the 

 optical behavior of crystals, though the clear color usual to olivine is 

 mottled by decomposition products. Next to the centres are radiated 

 fibrous masses of serpentine which give beautiful green colors between 

 the Nicol prisms, but the outsides of the crystals have been apparently 

 entirely removed, and the spaces filled with a mixture of calcite, quartz, 

 and pyrite. The pyrite is in cubes, and some of the crystals have 

 dodecahedral planes. One of these crystals of olivine is shown in Fig. 

 4 on PI. 9. This is simply another form of the alteration which con- 

 stitutes so large a part of the study of our basic eruptive rocks. 



I call attention here to the fact that the gabbros, which are described 



