442 REPOET ON NATIONAL MUSEUM, 1886. 



chlorite, talc, staurolite, magnetite, i)yrite, tourmaliue, aud rutile. 

 Through an increase in the amount of hornblende, chlorite, or talc, the 

 rock passes into hornblende, chlorite, or talc schist. 



Owing to their schistose structure and poor working qualities the 

 schists are but little used for architectural purposes, as already noted. 

 One of the most important of these rocks at present worked in this 

 country is the biotite schist near Washington, D. C. This is quite ex- 

 tensively quarried, though in a crude and itinerant manner, both in the 

 District of Columbia and on the opposite side of the Potomac Eiver, in 

 Virginia. The rock is as a rule fine grained and compact, and of a blue- 

 gray color, whence its popular name of "Potomac bluestone." It is at 

 times scarcely at all schistose, and contains a very considerable i)ropor- 

 tion of feldspar, thus approaching gneiss in composition. Several im- 

 l^ortant structures have been made of this stone, including George- 

 town College and one or two churches. It can be worked, however, 

 with great difficulty, and it is only by taking advantage of the natural 

 joint faces that it can be utilized with any degree of economy. Pyrite 

 is very abundant iu certain jiortions of the rock, and shows its utter 

 unreliability by retaining its bright, brassy luster unchanged in some 

 cases for many years, while in others it oxides almost immediately.* 



In Cape Elizabeth, Maine, near Portland, there occurs a fine-grained 

 talcose schist which is peculiar for the readiness with which it breaks 

 out into jointed blocks of about the right dimensions for building- 

 By taking adv^antage of this jointing several churches and other build- 

 ings in Portland have been erected and present a respectable appear- 

 ance, though through the oxidation of the included pyrite the walls are 

 stained almost beyond recognition. These joints are as sharp and clean 

 as though cut with a knife, and are usually indistinguishable in tlie 

 <iuarr3% having been recemented by calcite. A few blows from a ham- 

 mer on the end of a block will, however, almost always cause joints to 

 open, and often in very unexpected places. 



In the town of Bolton, Worcester County, Mass., there occurs a mica 

 schist that has been quarried for many years to furnish flagging ma- 

 terials for Hartford and other New England cities. The rock is fine 

 grained, distinctly schistose, and evenly laminated 5 it therefore splits 

 out readily into thin plates eminently suited for the purposes to which 

 it is applied. 



* It is possible that both ordinary pyrite and the gray variety, marcasite, are present 

 in these rocks, and that it is the latter mineral that so readily oxidizes, while the py- 

 rite remains unchanged. 



