122 The America)} Geologist. August, 1S95. 
^iude sur le Metamorphisme de contact des roches volcaniques. Par 
A. La Croix. (Mem. Acad. Sciences de I'lnstitut de France. Ex. de Tome 
XXXI, 1894.) The author confines himself to the consideration of vol- 
canic rocks j>?r sc, not older than the Tertiary, which obviously leaves 
unstudied a lai'ge field of contact metamorphism. It conduces to clear- 
ness and proV)ably to the correctness of his conclusions to embrace in 
the discussion only such phenomena as can be referred vithout doubt 
to the action of contacting volcanic rock, for many of the older volcanic 
rocks, and the sediments they modified, have undergone later modifica- 
tions through the action of other forces, and these changes are liable to 
be confounded with those which are due to the volcanic contact. 
The author considers separately the effect of basaltic and of trachytic 
rocks on other rocks in contact with or enclos3d in them, giving a his- 
tory of all researches on metamorjjhism and adding new results of his 
own. Among his conclusions are the following : 
1. Heat alone is unable to produce the phenomena of intense meta- 
morphism, but mineralizing waters under pressure have played an im- 
portant role in metamorphic changes. 
2. Volcanic rocks, either in outflow or in dikes, whatever the nature 
of the molten mass, produce identical effects upon the rocks with which 
they come in contact. 
3. Basic volcanic rocks, when they entirely enclose foreign masses, 
through heat cause a slight chemical transformation in a narrow zone, 
b^ the intimate mixture of the modifying and the modified rocks ; but 
trachytic eruptives produce chemical phenomena throughout their en- 
closures, the simple calorific phenomena seen in the basic eruptives be- 
ing developed as chetuical change through the action of caustic fluids 
under pressure and at a high temperature. 
4. These chemical transformations, so far as studied, are all produced 
by the addition of elements to the rock modified : such as vapor of wa- 
ter, alkaline silicates, chlorides, and sometimes fluorides, all of which 
have borne important parts in the reactions which have taken place. 
The number of minerals that can be formed, for instance in a limestone, 
by these reagents are necessarily quite limited, from which fact it is 
easy to see how the chemical transformations in contact metamorphism 
are always the same whatever be the nature of the modifying eruptives. 
5. In the case of enclosures of rock by eruptives, the greater chemical 
effect of the trachytic eruptives is probably due to their less fusibility, 
less conductivity of heat, greater porosity, through which they main- 
tain their mineralizing agents longer and are enabled to produce on their 
enclosed masses more profovmd mineralogical changes. 
6. The principal factor in contact metamorphism is not, therefore, so 
much the heat itself as the physical conditions in connection with which 
the heat operates. In one set of conditions (volcanic rocks) the miner- 
alizing fluids are readily disengaged, and in another (intrusives) they act 
energetically under pressure. • n. h. \v. 
£tude miiidralogique de la IherzoUte den PyHu^es et de ses phenom- 
tnes de contact. Par A. La Croix. (Extrait des Nouvelles Archives du 
