W. M. Fontaine—Mesozoie Strata of Virginia. 29 
Azoic. The force producing the sinking along these lines 
was plainly a lateral thrust, which at least in the case of the 
Richmond belt, acted from east towest. The sinking seems 
_ to have commenced at or near the close of the Permian period, 
and to have continued till toward the close of the Jurassic. 
_ The depression was accompanied by, in many cases, extremely 
_ rapid sedimentation, and toward its end, produced a rupturing 
_ of the crust and an outpour of fused rock. 
Wherever I have had an opportunity to examine carefully 
| the Azoic rocks bordering the Mesozoic belts, I find them pen- 
_ etrated by dykes of true igneous matter, such as felsite, granite, 
diabase, ete., which are much older than the Mesozoic beds. 
_ These dykes run parallel with the Mesozoic belts, and are con- 
_ fined to their vicinity. Their presence indicates that in the 
_ general metamorphism of the country, fracturing of the crust 
_ took place, and the metamorphic action was excessive along 
_ certain lines. There is no doubt in my mind that this previous 
_ weakening of the crust in definite belts, has much to do with 
_ the subsequent emission of the Mesozoic trap rocks in such well 
defined areas as we find to exist. I find also here a good 
example of the application of Von Richthoven’s conclusions 
_ concerning the order of precedence, and the association of igne- 
_ ous rocks. 
Lcopography.—The topography of the Azoic, and the included 
_ Mesozoic areas, is very significant, and may be studied to great 
advantage in Virginia. What I shall say under this head is 
more particularly applicable to the district which is limited on 
_ the north by the Potomac, on the south by the Appomatox, 
on the east by the Tertiary, and on the west by the Catoctin 
_ Tange of mountains. I apply this, the Maryland name, to that 
_ more or less connected range which penetrates far into Virginia, 
under many different local appellations. It runs about fifteen 
. miles east of, and nearly parallel to, the Blue Ridge 
_ very gently undulating character of the surface, which is so 
a 
The first thing that strikes the observer in this district is the 
marked as to arrest the attention of the non-scientific, and to 
_ Induce speculation as to the causes producing it. The country 
_ from the Catoctin eastward to the Tertiary, is a gently undulat- 
_ ing plain, descending from about 500 feet in elevation to the 
_ level of tide. Unlike the country near the Blue Ridge, and 
_ farther west, the topography is almost uninfluenced by the 
_ Structure and composition of the underlying rocks. In the area 
In question we have strata showing all gradations of hardness, 
_ With all degrees of proneness to decomposition, and dipping at 
_ Various angles, often steeply, yet all are planed down to a uni- 
_ form level. The streams, except the smallest creeks, cross the 
- Strike of the strata nearly at right angles, and are hardly at all 
