Quartz-Porphyry

Older

Forms.—Many of the older quartz-porphyries which occur in Palaeozoic and Pre-Cambrian rocks have been affected by earth movements and have experienced crushing and shearing. In this way they become schistose, and from their felspar minute plates of sericitic white mica are developed, giving the rock in some cases very much of the appearance of mica-schists. If there have been no phenocrysts in the original rock, very perfect mica schists may be produced, which can hardly be distinguished from sedimentary schists, though chemically somewhat different on account of the larger amounts of alkalis which igneous rocks con tain. When phenocrysts were present they often remain, though rounded and dragged apart while the matrix flows around them. The glassy or felsitic enclosures in the quartz are then very sug gestive of an igneous origin for the rock. Such porphyry-schists have been called porphyroids or porphyroid-schists, and in America the name aporhyolite has been used for them. They are well known in some parts of the Alps, Westphalia, Charnwood (England), and Pennsylvania. The halleflintas of Sweden are also in part acid igneous rocks with a well-banded schistose or granulitic texture.

The quartz-porphyries are distinguished from the rhyolites by being intrusive rocks. All Tertiary acid lavas are included under rhyolites. (J. S. F.) QUASSIA, the generic name given by Linnaeus to a small tree of Surinam (Quassia amara), superseded for medical pur poses in 1809 by the bitter wood or bitter ash of Jamaica, Picraena excelsa, which has similar properties and can be obtained in larger pieces. Since that date this wood has continued in use

in Britain under the name of quassia to the exclusion of the Surinam quassia, which, however, is still employed in France and Germany. Picraena excelsa is a tree 5o to 6oft. in height, and resembles the common ash in appearance. It is found also in other West Indian islands, as Antigua and St. Vincent. Quassia amara is a shrub or small tree belonging to the same natural order as Picraena, viz. Simarubaceae, but is readily distinguished by its large handsome red flowers arranged in terminal clusters. It is a native of Panama, Venezuela, Guiana and northern Brazil. Jamaica quassia is imported into England in logs several feet long and often nearly one foot thick. The wood is nearly white, has a pure bitter taste, and is without odour or aroma. It is usually met with as turnings or raspings, the former being obtained in the manufacture of the "bitter cups" which are made of this wood. The chief constituent is a bitter neutral principle known as quassin. It exists in the wood to the extent of about It forms crystalline needles soluble in alkalis, chloroform and 200, parts of water. There is also present a volatile oil. The wood contains no tannin, and for this reason quassia, like chiretta and calumba, may be prescribed with iron. The infusion is useful as a bitter tonic—a group of substances of which calumba is the type—and is also a very efficient anthelmintic for the thread worm (Oxyuris vermicularis). It is a substitute for hops.