DISCOVERY OF MARINE JURASSIC ROCKS IN 
SOUTHWESTERN. TEXAS. 
SEVERAL announcements of marine Jurassic rocks in New 
Mexico and Texas have been made by authors, the earliest as 
also the latest by Mr. Jules Marcou; but the marine sediments 
hitherto called Jurassic in these states belong to the Comanche 
series. Modtola jurafacies, Homomya jurafacies, Exogyra hilh, and 
possibly one or two other members of the fauna of the latter 
series, which are more or less clearly analogous with fossils of 
the European Jurassic, should doubtless be regarded as survivals 
from a preceding age. Such survivals are only to be expected, 
and these therefore do not contradict the results arrived at by 
Professor R. T. Hilland Mr. F. H. Knowlton, who have shown that 
the lowest formation of the Comanche series presents a Wealden 
fauna and flora. The data of old-world stratigraphy seem to 
show that the Wealden formation is part of the Cretaceous system, 
or, more definitely, is the estuarine and arenaceous extension of 
lower Neocomian sediments that are elsewhere of purely marine 
origin and largely calcareous. 
It is a principle recognized by many geologists that where 
the conditions afford only paleontological data for correlation 
and these data show a commingling of fossils of two successive 
systems, we should not suppose that the latest occurrence of 
fossils of the earlier system characterizes the highest rocks of 
that system, but should assume that the first appearance of a 
fauna essentially characteristic of a later system, whether it be accom- 
pamed by survivals from an older fauna or not, marks the beginning 
of a new rock-system and age. By this criterion Professor C. S. 
Prosser of the United States Geological Survey has recently drawn 
the line separating the Permian system from the Carboniferous in 
the Plains region; and in ‘accordance with the same principle, 
if the prevalent European acceptation of the Cretaceous system 
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