ROCKS FROM NORTHERN AND CENTRAL CHINA. 387 
to fragments of ordinary limestone, and in some cases they are composed 
very largely of bits of fossil shells lying in parallel seams, precisely as in 
common limestones. 
The third hypothesis we regard as improbable because of the lack of 
organic structure and of distinctive organic forms. ‘There is nothing in our 
specimens which suggests that the pebbles are either individual fossils or 
fragments of them.* If organic at all it seems necessary to suppose that 
they were formed by colonies of minute protozoa or algee whose forms have 
not been preserved. In the absence of any trace of organic structure the 
idea of such an origin must be regarded as purely speculative. It is a 
somewhat remarkable fact that all the conglomeratic limestones described 
in earlier papers belong to Cambrian terranes. It has been urged on this 
basis that the pebbles had the same stratigraphic significance as fossils and 
presumably were produced by organisms which flourished only in Cambrian 
times. Good examples of conglomeratic limestone, not essentially different 
from the Cambrian types, are now known from the lowest Algonkian terranes 
(Lower Huronian) of Michiganyt, and from the Ordovician of Belle Isle, 
Newfoundland, Pennsylvania, and southern Wisconsin. Brief references to 
“‘concretionary limestones”’ in the Belt formation (Algonkian) of Montana§ 
suggest a second observation of limestone conglomerate in the Pre-Cambrian 
rocks. It becomes evident that these pebbly limestones are by no means 
confined to the Cambrian, but that on the contrary they have a large 
range in geologic time. We must, therefore, regard their stratigraphic 
significance as apparent rather than real. 
Upon the basis of the study of isolated specimens it appears entirely 
plausible to suppose that the rocks are ordinary clastic conglomerates, the 
contents of which were derived from the erosion of limestones unassociated 
with other rocks. Nothing in the detached specimens themselves is incon- 
sistent with this view. The objections which have been raised are strati- 
graphic rather than petrographic. These conglomerates have not been 
found upon unconformable contacts, but recur at intervals through a thick 
series of beds, and the individual strata show remarkable uniformity in 
thickness over large areas. True clastic conglomerates derived from the 
* Dr. Theo. Lorenz in discussing the origin of these Chinese conglomerates makes the unreserved 
assertion that ‘‘they are not of mechanical genesis, but organic.’’ He first advanced the idea that they 
were produced by calcareous alge, but later abandoned it and suggested the possibility that sponges 
were concerned in the production of them. The results of his researches seem, therefore, to be incon- 
clusive. (Loc. cit.) 
+ Undescribed material from Kona dolomite of the Marquette district (specimen No. 45872 of the 
U.S. Geol. Surv. collection). 
t Leith, C. K., unpublished notes, 1905 (specimen No. 46267, U. S. Geol. Surv. collection). 
§ Peale, A. C., Paleozoic Section in the vicinity of Three Forks, Mont. Bull. U. S. G. S. 110, p. 17. 
