STRATIGRAPHY OF CHI-LI AND SHAN-SI. 145 
IGNEOUS INTRUSIONS IN THE TA-YANG LIMESTONES. 
There is a notable absence of igneous rocks in connection with the 
a-yang limestone throughout the area explored in western Chi-li. Near the 
overthrust which brings the gneiss up against the limestone northwest of 
Wan-hién, several small dikes of white aplite and fine-grained granite have 
been intruded into the sedimentary rocks. Along the contact with these 
dikes the limestone contains abundant crystals of pale amphiboles such as 
tremolite. In addition to these occurrences a small dike of brown-gray 
quartz-porphyry was found by Willis in the Ta-yang, a few kilometers north 
of T’ang-hién. No basic intrusives nor large batholites of any kind were 
observed. 
CORRELATION OF THE HU-T’O AND TA-YANG. 
In the general statement regarding the Ta-yang we have already 
stated our opinion that the limestone is to be correlated with the Hu-t’o 
system. Both are late Algonkian terranes, which have not been notably 
metamorphosed or intensely folded. Although the Ta-yang limestone has 
not been found in contact with the Wu-t’ai schists, there can be no rea- 
sonable doubt that, like the Hu-t’o, it is very much younger. Both the 
Ta-yang and Hu-t’o are unconformably, but immediately, overlain by the 
Sinian. 
The Hu-t’o system is believed to consist of a lower slaty group, in 
which thin limestones occur. The Ta-yang, however, is calcareous through- 
out and is lithologically similar to the limestone of the upper group of the 
Hu-t’o system. Between the localities at which we saw the two lime- 
stones there is a distance of 50 miles, 80 kilometers. The lithologic resem- 
blances and differences are such as are consistent with equivalency, but they 
are not sufficient to establish it. The Ta-yang limestone may represent 
the whole of the Hu-t’o system, or only the upper limestone group; or it 
may possibly lie between the latter and the Sinian. The two groups, how- 
ever, range themselves among the latest Pre-Cambrian formations, closely 
resembling the late Algonkian rocks of the northern Rocky Mountains in 
Montana, and we regard it as improbable that they lie in succession rather 
than as equivalents. 
It is not improbable that the Ta-yang limestone is the more uniform 
representative of the partly heterogeneous shore formations, which we 
distinguish as the Téu-ts’un and Tung-yti groups. 
There can be little doubt that the Ta-yang limestone is the exact 
equivalent of that observed by von Richthofen in the Nan-k’ou section 
and described by him as ‘‘Untersinisch.”’ 
