BUILDING AND ORNAMENTAL STONES. 305 



loosely compacted mass and the interstices filled by a cement composed 

 of an amorphous mixture of iron oxides, carbonate of lime, and clayey 

 matter. These are represented in black in the figure. It will be ob- 

 served that only the quartzes and a few of the feldspars are in a fresh and 

 uudecomposed condition, nearly all of the latter being badly kaolinized. 

 The Potsdam stone (Fig. 5) shows, however, a markedly different struct- 

 ure. Here the granules are wholly of quartz, and very much rounded 

 in form. No feldspars, mica, or other minerals are present. The orig- 

 inal rounded outline of the quartz granule is shown by the dotted 

 lines and deeply shaded portions, while every portion of the interstices 

 is occupied by a clear, colorless, siliceous cement binding the rock into 

 a hard, compact, and impervious quartzite almost absolutely unaffected 

 by chemical and atmospheric agencies.* 



The cause of the wide variation in relative durability of stones of 

 these two types becomes now at once apparent. In the first case the 

 abundant amorphous cement is not only slightly soluble, and liable to 

 partial removal by the water from rains, but it also facilitates the 

 absorption of a proportionally large amount of moisture. On being 

 subjected to repeated freezing and thawing while in this saturated 

 condition, the grains gradually become loosened and the characteristic 

 scaling results. Stones of the Potsdam type, on the other hand, are 

 practically non-absorptive and insoluble, and are susceptible to no other 

 natural influences than the constant expansion and contraction caused 

 by changes in temperature. They are consequently vastly more dura- 

 ble. Unfortunately they are also much harder, and hence can be 

 utilized only at greatly increased expense. 



(3) STATE OF AGGREGATION. 



This is one of the most important properties of building-stone, since 

 is dependent upon it very largely the hardness or softness of a rock 

 and its consequent working qualities. Many rocks composed of hard 



* This rock shows to beautiful advantage the secondary enlargement of quartz 

 granules by deposition of interstitial silica having the same crystallographic orien- 

 tation as the granules themselves, a peculiarity first noted by the Swedish geologist 

 Tornebohm, later by Sorby (Quar. Jour. Geol. Soc, 1880, p. 58), and since described 

 in great detail in American rocks by Irving and Van Hise, (Am. Jour, of Sci., June, 

 1883 ; also Bull. No. 8, U. S. Geol. Survey). I may say further here that the red and 

 brown colors of our Triassic sandstones seem to be due not merely to the thin pelli- 

 cle of iron oxides with which each granule is surrounded, but the feldspathic grains — 

 often badly decomposed — are stained throughout by the same material, and which 

 also occurs mixed with clayey, calcareous and silicious matter forming the cement. 

 This is never the case, so far as I have observed, in the Potsdam stones, in which the 

 oxide occurs only as a thin coating around each granule, as shown by the shaded por- 

 tions in Fig 5. My own experience, also, is to the effect that the fragments, of which 

 the Triassic stones are composed, are much less rounded by attrition than seems 

 ordinarily supposed, or as they are represented when figured. Fig. 4 is very typical 

 of the Portland stone, but it does not in the least resemble that given in Fig. 6, 

 Plate xn, Lith. & Min. of New Hampshire. Naturally, however, samples selected 

 from different beds, or from different localities, will be found to vary greatly. 



H. Mis. 170, pt. 2 20 



