tillates to suitable underground
reservoirs is responsible for the accumulation of oil and gas pools.

Oil and gas are distillates from these oil shales and asphaltic
deposits, and also from other organic sediments such as carbonaceous
limestones. The distillates have migrated to their present positions
under pressure of ground-waters. The stratigraphic horizons favorable to
their accumulation are generally recognized. The geologist is concerned
in identifying these horizons and in ascertaining where they exist
underground. He is further concerned in analysis of the various
structural conditions which will give a clue to the existence of local
reservoirs in which the oil or gas may have been accumulated. So
capricious are the oil migrations that the most intensive study of these
conditions still leaves vast undiscovered possibilities.

ANAMORPHISM OF MINERAL DEPOSITS

Mineral deposits formed in any one of the ways indicated above may
undergo repeated vicissitudes, both at the surface and deep below the
surface, with consequent modifications of character. They may be
cemented or replaced by introduction of mineral solutions from without.
They may be deformed by great earth pressures, undergoing what is called
dynamic metamorphism (pp. 25-27), which tends to distort them and give
them schistose and crystalline characters. They may be intruded by
igneous rocks, causing considerable chemical, mineralogical, and
structural changes. All these changes may take place near the surface,
but on the whole they are more abundant and have more marked effects
deep below the surface.

In general all these changes of the deeper zone tend to make the rocks
more crystalline and dense and to make the minerals more complex.
Cavities are closed. The process is in the main an integrating and
constructive one which has been called _anamorphism_, to contrast it
with the disintegrating and destructive processes near the surface,
which have been called _katamorphism_ (see also pp. 27-28). There is
little in the process of anamorphism in the way of sorting and
segregation which tends to enrich and concentrate the metallic ore
bodies. On the contrary the process tends to lock up the valuable
minerals in resistant combinations with other substances, making them
more difficult to recover in mining. Later igneous intrusions or the
ordinary ground-waters may bring in minerals which locally enrich ores
under anamorphic conditions, but these are re