774 REVIEWS 
beds of that age in the state, a comprehensive though brief treatment 
of the stratigraphy, sediments, and geologic history of the formations _ 
furnishes more than a mere introduction and background for the paleon- 
tology. The volume combines the observations, experience, and knowl- 
edge of the three geologists, who more than anyone else have for many 
years been occupied with the study of these formations, William Bullock 
Clark, Arthur B. Bibbins, and Edward W. Berry. 
In Maryland the Lower Cretaceous consists of three mutually 
unconformable formations, Patuxent, Arundel, and Patapsco, which 
collectively compose the Potomac group. For the benefit of those who 
may not have followed the progress of Atlantic Coastal Plain geology, 
it may be stated that no doubt is left as to the Cretaceous age of the 
oldest bed of the Potomac group. According to the conclusions of the 
authors the earliest Lower Cretaceous of the Atlantic coast in Maryland 
and Virginia was, after the long post-Newark hiatus, laid down in an 
old estuary or behind certain obstructive barriers. In this way they 
seem to compromise between the theory of California-gulf conditions of 
deposition proposed by McGee, on the one hand, and the absence of all 
types of marine life on the other. Account is, however, taken of the 
thinning of the beds to the eastward beneath the Tertiary overlap, 
as shown by drill records; of the fluviatile or lacustrine aspect of part 
of the Patuxent sediments; of the ancient forest soils and swamps of 
the Arundel; of the indications of differential warping, and of probable 
faulting near the “‘fall line.” The average thickness of the Potomac 
in Maryland is between 600 and 700 feet and the average of the variable 
dips about 60 feet to the mile southeastward. 
The Patuxent (lowest) formation, comprising a maximum thickness 
of 350 feet of cross-bedded sands and gravels, with some clays and 
kaolinized feldspar, is regarded as having been laid down in an estuary 
in a climate considerably warmer than that of today, within a region 
clothed with temperate rain forests, made up of a dense growth of ferns 
and cycads in more or less pure stands, with occasional conifers towering 
above the general level of the vegetation, which was relatively low, and 
gradually predominating in passing from the coast to the uplands. 
Growth rings in the petrified woods show seasonal changes, but the 
great width of the active growth ring and the narrowness as well as the 
irregularity of the zones of restricted growth suggest the occurrence of 
dry seasons rather than of frosty winters. It is certain that the winter 
cold was less than that of Maryland today. With the exception of one 
fish, the known Patuxent fossils are exclusively vegetal. They comprise 
