W. M. Fontaine—Mesozoie Strata of Virginia. 151 
logically related have been microscopically examined by E. S. 
Dana,* and some were analyzed by myself.+ It was shown 
that these large dikes which are so characteristic of the Mesozoic 
red sandstones of this coast are, wherever found, essentially 
uniform in composition and mineral constituents. They are 
compounds of labradorite, augite and magnetite, and vary only 
in the extent of their alteration. Professor Dana has concluded 
from this uniformity and their wide distribution over the At- 
lantic slope from Nova Scotia to North Carolina, that the dikes 
reach to profound depths. 
It is most probable that the large and small dikes that are so 
common among the crystalline rocks of New Hampshire, oceup 
fissures which were made during the elevation of the mountains. 
In the process of elevation, variable conditions must have been 
introduced in the strata at different places and times, on account 
of the conversion of mechanical work into heat, as has been 
shown by Mallet and others, and this would have modified the 
depth at which fused materials would be found beneath the 
surface. If partial crystallization took place before eruption, as 
in the case of many modern voleanic rocks, very variable con- 
ditions might also have been introduced at different times for 
their solidification. The Mesozoic sandstones referred to do 
not occupy a position that indicates a great strain upon the 
earth’s crust at the time of fracture, but are found in areas of 
upon elevation might fuse sedimentary deposits at various 
depths, and produce fissures that would be filled with the 
Art. XIX.—WNotes on the Mesozoic Strata of Virginia; by 
Wma. M. FonraIne. 
[Continued from page 39.] 
Hanover Area.—This, the northern portion of the Richmond 
belt, shows considerable differences from the Richmond coal 
field. The structure is not that of a basin, but it contains one 
or more broad anticlinals, the beds on the west side dipping 
northeast and east dip under the Tertiary. We have here also 
two series. The lower has the general character of the lower 
* This Journ., III, viii, 390. 4 Ibid, IIT, ix, 185. 
