IHIII.DING AND ORNAMENTAL STONES. 303 



scope. lu such cases it is customary amouj;' lithologists to griud a small 

 chip of the rock so thin as to be trauspareut, aud then, when properly 

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

 method many im[)ortant 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 melliod. By it 

 the presence of many minute ami perliaps important ingredients is 

 made known whose i)resencc would otherwise be unsuspected. This 

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

 the descriptions of the various kinds of rocks. 



In Fig. 1 of n. II is shown the structure of the muscovite biotite 

 granite of Hallowell, ]\Le., drawn as are the other figures on this plate 

 from thiu sections and under a magnifying power of about twenty-flvi' 

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

 [1) orthoclase, (2) microcline, (;>) 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, and occasionally small gar- 

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

 moreover, is pierced iu every direction bj^ minute hair-like crystals 

 which are supposed tq be rutile. The structure, as in all granites aud 

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

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

 mutual interference in jn-ocess of formation. Although the rock con- 

 tains a very large proportion of the liard minerals quartz and feldspar, 

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

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

 worli; much more readily and will crush under less pressure than those 

 of which Fig. 4 is a type. 



In Fig. 2 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 fornnitiou of this stone was the deposition of 

 concentric coating of lime about a nucleus which is sometimes nearly 

 round, but more frequently ({uite angular aud 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 

 l)ortions are nearly always amorphous the nuclei (and always the in- 

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

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

 tain of the oolites present no distinct concentric structure, but aj)pear 

 as mere rounded masses merging gradually into the crystalline interstitial 

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

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

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



