464 EEPORT ON NATIONAL MUSEUM, 1886. 



The stone lias already been alluded to under tbe head of sandstones. 

 It may rank as a fairly durable material, but contains clay bales and 

 other imperfections that unfit it for fine work of any kind. The Mu- 

 seum has received other samples of tufi's of various kinds from Cali- 

 fornia, New Mexico, Idaho, and Utah, but they are not at all used at 

 present, and their fitness or unfitness for any sort of building purposes 

 is a i)roblem for the future to decide. From near Phoenix, Ariz., has 

 been received a tuff consisting only of the firmly compacted shreds of 

 vokjanic glass or pumice and that is stated to have been used locally to 

 some extent.* 



Although so little used in this country, tuffs are very generally em- 

 ployed for building purposes in many foreign localities. They are 

 found abundantly in the volcanic districts of central France, and in the 

 Haute-Loire, where they have been used in the construction of churches 

 and dwelliug-liouses. The so-called " x)eperino " of the campagna of 

 Eome and Kaples, is a tuff formed by the consolidation of volcanic 

 ashes, and has been used in some of the buildings of these cities. It 

 was also used in the construction of the houses of Ilerculaueum and 

 Pompeii, t 



Khyolite tuffs are, as I am informed by Signor Aguileria, very largely 

 used for general building in certain i)arts of Mexico, the climate being 

 such as to render almost any material very durable. There is now a 

 large collection of these stones in the National Museum. 



(3) AKGILLACEOUS FRAGMENTAL ROCKS. THE SLATES, 

 (a) Composition and Structuhe. 

 Ordinary clayslate consists of consolidated clay. It is therefore 

 classed as a fragmental rock, although microscopic examination has 

 shown that it frequently contains crystalline matter, and that the rocks 

 pass by insensible gradations into what are called argillitic mica schists. 

 Mici^scopic examination of slates from Littleton, N. H., by Hawes,| 

 showed them to consist of a mixture of quartz and feldspar in frag- 

 ments as fine as dust. There is also present a " considerable quantity of 

 some amorphous coaly matters," and many little needles of a brightly 

 polarizing substance which is probably mica. The claj^slate of Han- 

 over, N. H., was found by the same authority to contain many minute 

 crystals of garnet and staurolite. An examination of some clayslates 

 from the Huronian region of Lake Superior, by Wichmann,§ showed 

 them to consist of a ''colorless isotropic gronndmassin wliich the other 

 constituents are apparently imbedded, Avhilst throughout are found 

 dust-like particles of a deep gray color, Avhich represent the chief con- 

 stituent, aiul consist i)robably of clay substances, the greater part ot 

 them probably of kaolin." Besides these constituents there were also a 

 few quartz and feldspar particles, scales of hydrated oxide of iron, flakes 



* See Am. Jour. Sci., Sept., 1886, p. 1991. 



t Hull: Building aud Ornamontal Stones, p. 283. 



tGeol. of Now Hiimpsliire, Vol. in, p. 237. 



$ Quar. Jour. Geol. Soc, Loudou, xxxv, 1879, p. 158. 



