288 _ The Mesozoic Sandstone of the Atlantic Slope. [May, 
which forms the floor of the basin. It contains all the coal found 
in the field.” 
Mr. Heinrich’s careful column shows the lowest coal (not 
counting occasional bituminous and carbonaceous slates and 
sandstones) is found at 571 feet from the granite, the second at 
600 feet, the third and fourth at 655 feet. 
On this subject Mr. Heinrich’s remarks (p. 35) are very instruc- 
tive ; for even the variable distance noticed by the author between 
the granite floor and the coal could scarcely account for so great 
a discrepancy. The faults in the Midlothian region are conceded 
by all, and. Mr. Heinrich’s section presents a very rational view 
of them; but why “ borings cannot be relied upon” is not clear, 
though of course their value as guides decreases as the distance 
to the desired locality increases. One would suppose that 
they were all that could be relied upon. 
A very interesting note in regard to certain varieties of the 
Potsdam strata, connect with this horizon the “compact vitreous 
quartzites and peculiar sandstones which have the grains of sand 
imbedded in a white, non-plastic, argillaceous matter,” occurring 
a short distance above Harper’s Ferry. Such rocks are indeed 
noticed elsewhere in positions entitling them to be considered 
Potsdam, but if by this are intended the quartz fragments imbed- 
ded in crystalline schist which make the high bluffs at Harper’s 
Ferry on the Maryland side of the river, it is most interesting to 
note that they are strikingly similar to a great series composing 
the middle and western portions of the South mountain in Penn- 
sylvania underlying the Orthofelsites and schists of probable Hu- 
ronian age. Neither is the Scolithus a sure guide to the age of 
Potsdam in the opinion of all geologists. 
The clay deposits and their supposed origin in “ marshes within 
the Azoic area swept away in a general and extensive erosion,” 
present certainly a new if somewhat hazardous line of dynamo- 
geological speculation, as also does the evident leaning of the 
writer towards a glacial movement to account for deposits of 
schists, granites, &c., on grit. It is noteworthy that Prof. Fon- 
taine, Mr. Russell and others, each from his independent line of 
argument, arrives at the probability of a series of shallow and 
marshy beds to the south of the Mesozoic belts. 
But the most novel explanation of the paucity of snivaal life in 
the Trias and Jura is that which supposes this time to have been 
one of great cold, when a huge ice sheet was advancing eastward 
