670 ROY R. MORSE 



structure are noted. Various types of biotitic and hornblendic 

 schlieren become more and more abundant. These include rounded 

 and orbicular and more angular types, as well as irregular areas and 

 dike-like schlieren. Pegmatite and aphte dikes become more 

 numerous, and more joints appear. Finally the intrusive loses its 

 massive character entirely in a marginal zone characterized by the 

 extreme fracturing shown in the accompanying photographs. 



The outcrops here pictured are located in the granodiorite, 

 about 100 feet inside the contact with the metamorphic rocks of 

 the ''Bedrock Series," in the railway cut one mile below El Portal. 

 Jointage of similar character continues into the metamorphic 

 rocks of the contact zone, but in these heterogeneous rocks it is 

 not so regular and the analysis of the joints becomes more difficult. 



CHARACTER OF THE JOINTS 



As is evident from the photographs, the exposures are here cut 

 by three dominant sets of joints. These are intersected by numer- 

 ous less regular and less persistent fractures of variable extent. 

 What we may here consider as the master joints include the nearly 

 vertical set of dike joints and the two sets of moderately inclined 

 joints. 



The steep joints comprising the first set are often made con- 

 spicuous by the presence along the joint plane of thin sheets of 

 white aplite, varying from less than one-half inch in thickness up 

 to six inches. These aplite dikes are here practically restricted to 

 the system of steep joints, though here and there a dike may be 

 seen to break away from the main joint plane and follow a minor, 

 slightly divergent, subsidiary joint, as is well shown in the outcrop 

 of Figure i. The strike of the joint planes of these dike joints at 

 the locality studied is about N. 85° W.; the dip varies 5° either 

 way from the vertical, though most frequently it is about 85° to 

 the south. 



The two sets of inclined joints are somewhat stronger and more 

 regular than the dike joints. In the locality studied one of these 

 sets strikes N. 50° W. and dips about 35° to the south; the other 

 set strikes N. 15° W. and dips about 20° to the east. Though not 

 spaced with exact uniformity the effect of these intersecting fracture 



