ROCKS FROM NORTHERN AND CENTRAL CHINA. 383 
crystals of calcite and bits of green glauconite (?). Fragments of fossils are 
rather numerous and may be seen in the hand-specimen. 
In spite of its unusual peculiarities this variety is closely related to 
the last described. The ground-mass is of the same character; there are 
fossils here and there, and even a few brown corpuscles identical with those 
in No. 17. The only conspicuous difference between the two relates to the 
coarsely crystalline bodies of clear calcite, which give this variety its por- 
phyry-like appearance, and secondly, the large amount of siderite present 
in this specimen. In describing this rock it will, therefore, be sufficient 
to deal with these two features alone. 
The siderite occurs in the form of more or less perfect rhombs, loosely 
aggregated and cemented with finely granular calcite. In some parts of 
the slide the mineral is absent, but elsewhere it is abundant; and there is 
no line of demarkation between the two areas. Where calcite is the domi- 
nant mineral, fossils and even oolitic structures are fairly well preserved, 
but in the areas of siderite such features are very rare and imperfect. On 
this account it is believed that the calcite was originally present, but has 
been replaced by the iron carbonate. These rhombic crystals of siderite 
are bordered with earthy limonite, and it is this alteration product which 
produces the ocherous blotches which appear on the surface of the rock. 
The spheroidal bodies of calcite are about the size of the oolitic nodules 
in No. 17. Each body is made up of a single calcite crystal or of a few 
large interlocking grains (Plate LV, Fig. E). Grayish rhombs of dolo- 
mite are occasionally included in them, but siderite is much more abundant 
and in some cases has entirely replaced the calcite. Where this replacement 
has occurred the originally regular outline of the spherule has been much 
injured by the growth of the sharp angular crystals (Plate LII, Fig. A). 
The size and shape of these spheroidal bodies at once suggests that they 
were formerly oolitic nodules; but on first inspection there appears to be 
nothing to verify the surmise. In this slide, however, we have proof of the 
transition of the undoubted oolitic bodies into spheroids consisting of one 
or more large crystals of calcite and lacking all of the characteristics of 
oolites. Ina few of these bodies the process has not been completed and in 
one of them a crescent-shaped area, which is the remnant of a brown spherule 
precisely like those in No. 17, partially incloses a rounded body which 
consists of two to three large crystals of clear calcite (Plate LI, Fig. B). This 
specimen, therefore, represents a further advance in alteration beyond Nos. 
17 and 18, through crystallization of the corpuscles. It is a noteworthy 
fact, however, that in this case the crystallization of the ground-mass has 
not kept pace with the changes in the oolitic bodies, 
