TRACHYTIC FORMATION. 171 



10. The Tra'chytic formation is very extensive. It presents 

 itself not only in conical hillocks, running in narrow bands, but 

 also in piled-up tables on the surface ; tra'chyte constitutes great 

 mountains, most frequently united in very extended groups, which 

 form very high masses, ordinarily the highest in the country, 

 covered with asperities ; their sides are broken into valleys and 

 deep ravines, with steep escarpments, and with all the circum- 

 stances of lofty chains. The tra'chytic formation is in strong 

 contrast with the igneous rocks we have heretofore studied, al- 

 though close inspection would show them to bear various relations 

 with deposits of basa'lt or lava. 



11. The rocks which constitute the tra'chytic formation are ex- 

 tremely varied. Most of these substances, as their name indicates, 

 are rough to the touch, because they are most generally finely 

 porous, sometimes cavernous, scoria'ceous, pumice-like ; but there 

 are some that are perfectly compact, and present the porphyri'tic 

 structure, frequently with tints of grey, red, brown, or black, on 

 which are white crystals of albi'te and of rya'colite. There are 

 some, more or less earthy, ordinarily of clear tints, called domi'te, 

 oecause the Puy de Dome is composed of it. The base of all 

 these rocks, which is inattackable by acids, is albi'tic or ryacoli'tic, 

 formed of a multitude of microscopic crystals mingled together, 

 the whole constituting a mass which is more or less compact. The 

 disseminated substances are albi'te, in crystals of greater or less 

 size, rya'colite, black mica, amphibole hornblende, but rarely py'- 

 roxene augi'te. Cluartz in crystals, and chalcedony in small nodules 

 are also found in it sometimes, and especially in a certain very 

 cavernous species, hitherto found only in Hungary, the cement of 

 which also contains many small striated balls of sphe'rolite (from 

 the Greek spheira, a sphere, and lithos, a stone). 



12. The name pho'nolite (from the Greek phone, a sound, and 

 lilhos, a stone) has been given to rocks more or less analogous to 

 tra'chyte, but differing from it in this, that their base is attackable 

 by acids, leaving a residue of rya'colite. These rocks are most 

 often compact, greyish or greenish, sometimes porphyroid, but in 

 which disseminated substances are rare. They are frequently 

 divided into plates or leaves of variable thickness, and in certain 

 cases the whole mass is divided into prismatic columns, which are 

 more frequently divergent and contorted than vertical. Pho'no- 

 lites have been sometimes confounded with certain porphyro'idal 

 varieties of tra'chyte, which possess nearly the same appearance, 

 but not the same solubility. 



13. Some tra'chytic formations contain considerable deposits of 



10. Under what forms do we find the tra'chytic formation ? 



11. What are the characters of those rocks which constitute th* tnf 

 chytic formation ? What is domite ? 



12. What is phonolite ? What are its characters ? 



13. Do all tra'chytic formations contain obsidian? 



