Some Metamorphosed Eruptwes of Maryland. 157 
metamorphism. In the first it is without question, heat, while in the 
second we are forced to believe that heat, if a factor at all, is a very small 
one, its place being taken by pressure, accompanied by internal movement. 
Where the rocks show most disturbance in the field, there the microscope 
reveals the maximum of pseudomorphic change and alteration of texture. 
We see then, that much the same effect can be produced at least along 
two different lines, and from what has been said about the chemical com¬ 
position of the rock before and after the process, it would not seem to be 
impossible to produce by metamorphism from a shale and a gabbro, the 
same resultant rock, provided their analyses in toto were the same. 
The study of metamorphosed massive rocks has yielded perhaps the 
most satisfactory results, since the original character is frequently betrayed 
even after pronounced metamorphic action, by areal or structural relations, 
or by the remains of rock-textures. The sending of dikes or apophyses into 
surrounding rocks, the presence of included fragments of them, and 
the remains of crystal boundaries now existing for some other species 
known to be secondary, are valuable bits of evidence. In some re¬ 
gions where massive gabbro oi diabase occurs in conjunction with 
hornblende gneisses, it is seen that they pass insensibly into one an¬ 
other, and careful study has shown that the hornblende gneiss is the 
metamorphosed form of the gabbro. Geologists have further been able to 
trace the steps of the process by examining the rock from parts of a 
given area that have been but slightly effected, at localities where most 
disturbance has taken place, and at intermediate points. It would seem 
that the feldspars are the most sensitive to disturbance, a wavy extinction 
between crossed Nicols being noticeable when the other minerals show no 
effects. If the process is more advanced, a breaking up of this mineral 
may result, and one of the decompositions to saussurite, epidote or amphi- 
bole may be noticed. Pyroxene is altered to amphibole (Uralitization). A 
breaking up of hornblende crystals takes place with an arrangement of the 
longer axes of fragments parallel to a plane of schistosity. The details of 
the process differ somewhat according to the original chemical and min- 
eralogical composition of the rock, the intensity of the matamorphism, 
and other conditions which we can not analyze. 
A region which furnishes an example of the metamorphism of gabbro 
to hornblende-gneiss is that southwest of the city of Baltimore. The 
Potomac formation (Juro -Cretaceous) consists in this vicinity of unconsol¬ 
idated gravels which rest on the crystalline schists and gneisses of the 
Pre-Cambrian belt, and their included intrusive members. To the west 
and northwest of the city of Baltimore is a nearly circular area of hypers- 
thene-gabbro and gabbro-diorite, which has become well known through 
the beautiful studies of Dr. Geo. H. Williams. The results have been pub¬ 
lished in a bulletin of the U. S. Geological Survey. Professor Williams 
has shown that the hypersthene-gabbro of that area passes into gabbro- 
diorite by the alteration of both hypersthene and diallage to horn- 
