BUILDING AND ORNAMENTAL STONES. 303 



scope. Iii such cases it is customary among lithologists to grind a small 

 chip of the rock so thin as to be transparent, and then, when properly 

 mounted in Canada balsam, to submit it to microscopic study. By this 

 method many important points of structure and composition are brought 

 out that would otherwise be unattainable. The physical condition of 

 the minerals of a rock, their freedom from decomposition, and methods 

 of arrangement can often only be ascertained by this method. By it 

 the presence of many minute and perhaps important ingredients is 

 made known whose presence would otherwise be unsuspected. This 

 subject is further treated under the head of Bock-forming minerals and 

 the descriptions of the various kinds of rocks. 



In Fig. 1 of PL ii is shown the structure of the muscovite biotite 

 granite of Hallowell, Me., drawn as are the other figures on this plate 

 from thin sections and under a magnifying power of about twenty-five 

 diameters. This is a granite of quite complex structure, consisting of 

 (1) orthoclase, (2) microcline, (3) plagioclase, (4) quartz, (5) black mica, 

 or biotite, and (G) white mica or muscovite. There are also little needles 

 of apatite, scattering grains of magnetite, aud occasionally small gar- 

 nets present, which, however, do not show in the figure. The quartz, 

 moreover, is pierced in every direction by minute hair-like crystals 

 which are supposed to be rutile. The structure, as in all granites and 

 gneisses, is crystalline throughout, as in the marbles (Fig. 3) and diabase 

 (Fig. 4). The crystals are, however, very imperfect in outline, owing to 

 mutual interference in process of formation. Although the rock con- 

 tains a very large proportion of the hard minerals quartz aud feldspar, 

 these do not interlock so thoroughly as do the augite and feldspars in 

 the diabase. As, moreover, quartz is a brittle substance, these rocks 

 worn much more readily and will crush under less pressure than those 

 of which Fig. 4 is a type. 



In Fig. 1 of the same plate is shown the structure of an oolitic lime- 

 stone from Princeton, in Caldwell County, Kentucky. It will be noticed 

 that the first step in the formation of this stone was the deposition of 

 concentric coating of iime about a nucleus which is sometimes nearly 

 round, but more frequently quite angular and irregular. After the 

 concretions were completed there were formed in all cases about each 

 one narrow zones of minute radiating crystals of clear, colorless cal- 

 cite ; then the larger crystals formed in the interstices. An examina- 

 tion of the section in polarized light shows that while the concentric 

 portions are nearly always amorphous the nuclei (and always the in- 

 terstitial matter) is frequently crystalline. The nuclei are composed in 

 some cases of single fragments or, again, of a group of fragments. Cer- 

 tain of the oolites present no distinct concentric structure, but appear 

 as mere rounded masses merging gradually into the crystalline interstitial 

 portions. On the application of acetic acid to an uncovered slide of this 

 rock a brisk effervescence at once set in, which, when the slide was again 

 placed on the stage of the microscope, was seen not to arise from all 



