ROCKS FROM NORTHERN AND CENTRAL, CHINA. 467 
thereby impart to the rock its banded structure, compel the belief that it 
is the result of dynamic metamorphism. ‘This is further indicated by the 
intimate relations of the biotite to hornblende. In the case of one large 
hornblende which is not parallel to the banding, a large cavity in its side 
is occupied by a biotite scale which is oriented with reference to this struc- 
ture of the gneiss. The formation of biotite from hornblende in presence 
of potassic feldspars is known* to occur frequently under anamorphic 
conditions and is an important factor in the making of many schistose 
rocks. ‘The biotite itself has altered in this case to chlorite, the new mineral 
developing in narrow streaks parallel to the cleavage of the mica. At the 
same time fibers of rutile separate out, thus showing that the biotite was 
titaniferous (Plate LVI, Fig. B). Not infrequently epidote has also been 
produced as a result of these changes. 
Of the rarer minerals the iron ores are the most prominent. Magnet- 
ite and ilmenite are both common, while pyrite is rather rare. They occur 
in comparatively small grains or irregular bodies. ‘Titanite, although not 
abundant, is widely distributed and is sometimes found bordering masses 
of ilmenite. Apatite and zircon are present as in most of the granitic 
rocks. 
Whatever may be the history of this rock it now shows comparatively 
little evidence of having been severely metamorphosed. The roughly 
parallel streaks of mica are thought to have been developed under such 
conditions, and it is probable that the hornblende was the original ferro- 
magnesian mineral which provided much of the material to form this bio- 
tite. Aside from this, however, we can distinguish very few changes which 
occurred while the rock was subjected to the conditions which obtain in the 
zone of flowage; most of the other alterations observable are those which 
characterize the zone of katamorphism. The quartz grains, and to a less 
extent the feldspars, exhibit strong undulatory extinction, indicating that 
they have been severely strained, without being actually fractured. In 
only a very few places in the slide is it possible to observe fracturing or 
granulation of the minerals, and it is very doubtful if, barring the formation 
of the biotite, any considerable portion of the mass has recrystallized. 
PALEOZOIC. 
The Paleozoic rocks of the Yang-tzi region are sedimentary in origin. 
Not a single dike of igneous rock, either of this age or of any later period, 
has yet been found in the district. 
The stratified rocks consist of limestones and shales with occasional 
quartzitic sandstones; locally there are certain boulder clays which are of 
* Van Hise: A Treatise on Metamorphism, U. S. Geol. Surv. Monograph xLvu, p. 288. 
