370 A. KEITH OUTLINES OF APPALACHIAN STRUCTURE 



gins of most, if not all, batholiths show vast numbers of intruding dikes 

 and sheets which extend for long distances from the main body and are 

 of all dimensions, even down to inches. The actual flow of the material 

 is shown by the wavy banding near the contact (flow banding), by the 

 strings of inclusions along the contacts, and by the finer gTain of the 

 intrusive where it was chilled near the contacts. Therefore, since the 

 intrusive material was sufficiently fluid to move freely under the pres- 

 sures which affected it, it is clear that a hydrostatic condition jDrevailed 

 in the magma as a whole. That being the case, any pressure applied to 

 any considerable part of the magma was transmitted throughout its mass. 

 If the fluidity were perfect, as in water, the pressure would be equal 

 throughout the mass, but in proportion as the viscosity increased so the 

 pressure transmitted would diminish. Furthermore, whatever was the 

 direction of the initial pressure, the pressure transmitted would be at 

 right angles to all confining surfaces, if the fluidity were perfect. In 

 proportion as the fluidity was less, the direction of the transmitted 

 pressure would approach more nearly that of the initial pressure. In 

 the batholiths which have been observed a fluid condition is apparent, so 

 that there can be no serious doubt that the hydrostatic condition pre- 

 vailed, and that any pressure applied to any part of the batholith was 

 transmitted in large measure to all of its mass and further transmitted 

 at right angles to the confining w^alls. 



ATTITUDE OF ROCK PARTINGS 



In applying this conclusion to the problem in hand it is necessary to 

 consider what were the most likely attitudes of the confining surfaces of 

 the batholiths; in other words, in what shape would the countiy rock 

 break when the batholith was intruded. It is easy to see that the sedi- 

 mentary rocks would part freely along their layers and that schists and 

 gneisses derived from them would do the same. The same is true to a 

 less extent for the banded gneisses which have a layered condition, but 

 in which the layers are more or less interlocked by mineral crystals. The 

 massive rocks, like granite and cliorite, would be entered with the greatest 

 difficulty. In the granites of the Precambrian, however, which would 

 necessarily be traversed by the Devonian and Carboniferous granites, 

 there has been a large amount of shearing. This has caused many zones 

 of sheeting, and even of thin schists along which partings would easily 

 be made. Opportunity for intrusion of igneous material would thus be 

 far greater in layered or sheeted rocks than in others, so that they prac- 

 tically control the separation of the country rock in order that intrusion 

 may have taken place. 



