464 REPORT ON NATIONAL MUSEUM, 1886. 



The stone has already been alluded to under the head of sandstones. 

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

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

 seum has received other samples of tuffs 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 problem for the future to decide. From near Phoenix, Ariz., has 

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

 volcanic 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 beeu used in the construction of churches 

 and dwelling-houses. The so-called " peperino " of the campagua of 

 Rome and Naples, 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 Herculaneum and 

 Pompeii, t 



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

 used for general building in certain parts 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) ARGILLACEOUS FRAGMENTAL ROCKS. THE SLATES. 

 (a) Composition and Structure. 

 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 argiliitic mica schists. 

 Microscopic examination of slates from Littleton, N. H., by Hawes,J 

 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 clayslate 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 Wichmaun,§ showed 

 them to consist of a u colorless isotropic groundmass in which the other 

 constituents are apparently imbedded, whilst throughout are found 

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

 stituent, and consist probably of clay substances, the greater part of 

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

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



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



t Hull : Building and Ornamental Stones, p. 283. 



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



§ Quar. Jour. Gool. Soc, London, xxxv, 1879, p. 158. 



