438 REPORT ON NATIONAL MUSEUM, 1886. 



admirably adapted for polished columns, pilasters, aud other decorative 

 work. The lasting power of the norites, when polished, is yet to be 

 ascertained. After an exposure of untold years in the quarry bed the 

 surface has turned white. No data are obtainable for calculating their 

 lasting qualities in the finished structure. 



(3) MELAPHYR. 



The melaphyrs, as defined by Eosenbusch,* are massive eruptive 

 rocks, consisting of plagioclase, augite, and olivine, with free iron oxides 

 and an amorphous or "porphyry" base. They are thus of the same 

 mineral composition as the basalts and olivine diabases, but differ struct- 

 urally, and belong in great part to the Carboniferous and older Termian 

 formations. Although very abundantin many parts of the United States, 

 they are scarcely at all quarried owing to their dull colors and poor 

 working qualities. 



In the Brighton district of Boston, but a few miles out of the city 

 proper, and in other localities in the vicinity, there occur small outcrops 

 of a greenish or sometimes purplish melaphyr, or " amygdaloid," the 

 lithological nature of which was, I believe, first correctly stated by 

 E. R. Benton.f The prevailing color of the rock is greenish, often 

 amygdaloidal, the amygdules being composed often of epidote, thus 

 spotting the surface with greenish-yellow blotches. The rock is greatly 

 altered, only the feldspars of the original constituents remaining now 

 recognizable, while chlorite, quartz, calcite, epidote, and several other 

 minerals occur as secondary products. The rock is nevertheless very 

 firm, compact, and durable, and is being quarried to some extent for 

 rough work. It would seem fitted for a yet wider architectural appli- 

 cation. 



(4) BASALT. 



This rock differs from diabase only in point of geological age, be- 

 ing a product of post-Tertiary eruptions. It is, as a rule, less perfectly 

 crystalline, still retaining portions of its glassy magma, and the surfaces 

 of the flows are often less compact owing to their having been exposed 

 to atmospheric agencies for a shorter period, and consequently having 

 suffered less erosion. Owing in great part to the fact that basalts occur 

 in this country only in the western and more recently settled portions, 

 as do also the andesites and rhyolites, they have been heretofore but 

 little utilized. There would seem, however, no reason for excluding the 

 rock from the list of available building materials in those regions where 

 it occurs in such form as to be accessible. At Petaluma, Bridgeport, and 

 other places around the bay of San Francisco there lie immense sheets 

 of this rock, but which are worked now only for paving materials. 

 Like the andesites and rhyolites the basalts will not polish, and their 

 colors are such as to exclude them from all forms of interior decorative 

 work. 



* Mik. Physiog. der Mussigen Gesteino, p. 392. 

 tProc. Bos. Soc, Vol. xx, p. 416. 



