304 REPORT ON NATIONAL MUSEUM, 1886. 



portions of the slide alike, but to be confined almost exclusively to the 

 outer non-crystalline portions of the oolites, so that in time these almost 

 completely disappeared, leaving the crystalline nuclei and cementing 

 material till the very last. Some of the outlines thus left are peculiarly 

 deceptive, having almost the appearance of a cross- section of coral or a 

 crinoid stem. This structure is common, so far as I have observed, to 

 all the oolitic limestones of both Kentucky and Indiana. In the weath- 

 ering of these stones then we would have produced an effect precisely 

 the opposite of that produced in fragmental siliceous rocks. In the 

 latter case the cement is removed and the grains themselves are but 

 slightly acted upon; in the former, the grains themselves disappear 

 and the cementing material remains. 



It should be remarked, however, that we have as yet no proof that 

 the action of au acid atmosphere on one of these oolites would proceed 

 with other than extreme slowness. In fact, their compactness, freedom 

 from cleavage, fractures, and flaws would seem to indicate just the con- 

 trary. Further investigations on this point are necessary before one can 

 speak definitely. 



The microscopic structure of ordinary white crystalline limestone is 

 shown in Fig. 3, drawn from a magnified section of a West Kutland 

 marble. The entire mass of the rock, it will be observed, is made up 

 of small calcite crystals of quite uniform size closely locked together, 

 and with no appreciable interspaces. The dark stripes across the crys- 

 tals are caused by twin lamella 1 and cleavage lines. All traces of its 

 fossil origin, if such it had, have been obliterated by metamorphism. 



Fig. 4 is that of a- diabase from Weehawkeu, N. J. The elongated, 

 nearly colorless crystals, shaded with long parallel lines, are a plagio- 

 clase feldspar, the very irregular ones augite, while the perfectly black 

 and opaque are magnetite. The figure is, however, given to show the 

 structure rather than the mineral composition of the rock. It will be 

 noticed that every portion of available space is occupied, there being 

 no residual spaces to be filled b3 7 cement, as in the sandstone ; also that 

 the feldspars and augites so closely interlock that they can not be forced 

 apart without breaking. As both of these minerals are quite tough 

 and hard, the great strength, durability, and hard-working qualities of 

 the rock can readily be understood, although the constituents them- 

 selves are not harder than those that go to make up some of the most 

 friable sandstones. 



As showing the differences in structure and composition of the sand- 

 stoues, Figs. 5 and G are given, drawn from thin sections of the brown 

 Tnassic stone from Portland, Conn., and a reddish Potsdam stone from 

 quarries in the town of Potsdam, N. Y. In the first mentioned, Fig. 6, 

 the stone, it will be noticed, is com posed of (1) clear, angular grains of 

 quartz, (2) clouded grains of orthoclase and plagioclase, the latter being 

 recognized by its parallel banding, and numerous irregular and con- 

 torted shreds of black and white mica. These are all crowded into a 



