ROCKS FROM NORTHERN AND CENTRAL CHINA. 473 
The sandy matrix consists largely of closely packed angular grains of 
quartz. The interstices between these veins are completely filled with a 
fine-grained granular cement of quartz, which has evidently been deposited 
in that situation from solution. The cement is discolored by yellowish- 
gray material in which limonite and minute scales of mica may be perceived. 
Many of the quartz grains show undulatory extinction, and some of them 
are crossed by fractures. Along the edges of the grains it is sometimes 
possible to see the first traces of granulation. Since the cement does not 
fill the fractures it seems probable that the mechanical deformation which 
these features indicate occurred at some time subsequent to the cementing 
of the sandstone. 
CARBONATE ROCKS. 
Dark fossiliferous limestone, No. 146.—This is a typical specimen of 
the dark limestone which overlies the red shales of the K’ui-chéu series 
along the lower course of the Ta-ning-ho. It was obtained 7 miles, 11 
kilometers, south of Ta-ning-hién, Ssi-ch’tian. 
A massive limestone of blackish-gray color. The bulk of the rock has 
an aphanitic texture, but glistening crystals of calcite are scattered abun- 
dantly throughout. Fragments of a Terebratuloid brachiopod are visible 
here and there. 
Under the microscope the aphanitic portion of the rock appears as a 
cryptocrystalline mass of calcite darkened by indistinguishable impurities. 
Nebulous spots, bits of shell, specks of carbon and limonite, and even well- 
preserved casts of foraminifera, are distributed through this dark material. 
The most prominent features of the rock are the abundant coarse crys- 
tals of calcite already mentioned. The bodies are rounded or subangular in 
form and vary from 0.2 to 1 mm. in diameter. In some cases distinct forms 
of minute shells can be seen inclosing the calcite bodies, but in the vast 
majority of cases the shapes are not recognizable. 
This rock has obviously been but slightly altered from its original 
condition; as proof of this we have the preservation of such delicate shells 
as those of the foraminifera. On this account we can hardly suppose that 
the calcite globules are recrystallized oolites, as they appear to be in some 
of the Cambrian limestones (No. 11); and in this connection it may be 
observed that no traces of oolitic structures can be detected in this speci- 
men. In many cases the calcite crystals occupy the cavernous portions 
of little molluscoid shells, while others are apparently crystallized bits of 
the shell material itself, and possibly all of them are thus connected with 
organic particles. 
