BUILDING AND ORNAMENTAL STONES. 



465 



of coal, minute tourmalines, and mica fragments. The Maine slates as 

 observed by the author contain quite large flakes of greenish mica, and 

 many quartz and carbonaceous particles. As a rule the dark color of 

 slate seems to be due to these carbonaceous particles, since they are 

 very abundant in the dark varieties, as those of West Bangor, Pa., and 

 almost entirely lacking in the light-greenish varieties, as those of Cas- 

 tletou and Fairhaveu, Vt. The red slates of Granville, N. Y., are made 

 up of a groundmass of impalpable red dust in which are imbedded in- 

 numerable quartz and feldspar particles all arranged with their longer 

 axes parallel with the cleavage direction of the slate. 



Although slate is undoubtedly a sedimentary rock, its remarkable 

 cleavage property is in no way connected with its bedding, as might 

 at first be supposed, but as shown by Sorby,* Daubree.f and others, 

 is caused by pressure acting in a direction at right angles with this 

 cleavage plane, and which may or may not correspond with that of its 



bedding. 



(&) Uses of Slate. 



Besides for roofing purposes, slates are used for billiard-tables, man- 

 tels, floor-tiles, steps, flagging, and in the manufacture of school-slates. 

 For the last-named purpose a soft, even-grained stone is required, and 

 almost the entire supply is at present brought from Pennsylvania and 

 Vermont. 



Of late years the business of marbleizing slates for mantels and fire- 

 places has become an important industry. All kinds of stones can be 

 imitated by this process, but that most commonly seen is the green verd- 

 antique marble and the variegated marbles of Tennessee. Like many 

 counterfeits, however, the work is too perfect in execution, and need de- 

 ceive none but the most inexperienced. 



The following table gives the various sizes of slate made for roofing, 

 and the number that are necessary for a "square," i. c, a space 10 feet 

 square, or containing an area of 100 square feet :| 



Size. 



No. of 



slates 



to a 



square. 



Size. 



No. of 



slates 



to a 



square. 



i 



Size. ^ 



No. of 

 slates 

 to a 



square. 



Inches. 

 24 by 14 



98 

 105 

 114 

 124 

 138 

 116 

 126 

 138 

 151 

 141 

 154 

 16!) 

 188 

 174 

 192 



Inches. 

 18 by 9 



213 



L'30 

 222 



240 ; 



247 1 

 316 i 

 300 1 

 327 i 

 374 j 

 436 ! 

 400 

 457 i 

 570 ! 

 640 



Inches. 

 10 by 7 



588 



24 13 



18 8 



10 6 



686 

 823 

 1, 039 

 600 

 686 



24 12 



16 10 



10 5 



24 11. 



16 9 



10 4 



2+ 10 



16 8 



9 8 . 



22 13 



16 7 



9 7.. 



22 12 



14 9 



9 6 





22 11 



14 8 



9 5 .. 



960 



22 10 



14 7 



14 6 



9 4 . 



1 200 



20 12 



8 6 



9(50 



20 11 



12 8 



8 5 



1,152 



20 10 



12 7 



8 4. 



1,440 



20 9 



12 6 



7 5 



1 440 



18 11 



12 5 



7 4 



1, 800 

 2,400 



18 10 



10 8 



7 3... 









* Edinburg Pliilosoph. Jour., iv, 1853, p. 137. 

 tGeologie Experimentale, p. 391. 



I From Rep. D 3, Vol. i, p. 142, Second Geol. Sur., Pennsylvania. 

 H. Mis. 170, pt. 2 30 



