The Geological Story Briefly Told 71 



Franciscan Rocks Much Folded and 

 Faulted 



The rocks of the Franciscan group of formation have been 

 folded and faulted in a most complex manner, and have been 

 penetrated at various times by dikes of igneuos rocks, in great 

 abundance and variety. In contrast with the younger rocks of 

 later formations the strata have been sharply folded, shattered, 

 and crushed together through mountain-making movements as 

 well as igneous intrusions. Volcanic lavas are inter-stratified 

 with the sedimentary deposits. These volcanic rocks occur in 

 sheets of moderate thickness that conform to the planes of 

 stratification. 



The Franciscan epoch of sedimentation was brought to a 

 close by the uplift and deformation of the region. A feature of 

 the disturbance was the intrusion into the Franciscan strata at 

 many places of basalt and diabase, followed by the intrusion of 

 other igneous masses. The masses of intrusive rock cut boldly 

 and irregularly across the strata of the sedimentary beds, whereas 

 the volcanic lavas occur in sheets that conform to the sedi- 

 mentary strata. The greater part of the igneous rocks asso- 

 ciated with the Franciscan sediments are not of contempora- 

 neous origin but were intruded after the formation of the 

 sediments. One of the results of the intrusion of the igneous 

 rocks was the metamorphism of rocks with which they came 

 in contact. 



The exact age of the Franciscan formations is in doubt. The 

 great disturbance of the strata due to upheaval, crushing, 

 crumpling and shattering of the rocks has largely destroyed 

 any fossil remains of plants or animals that may have been once 

 deposited in the sediments. Such evidence as there is makes it 

 possible to place the time of the invasion of the Franciscan sea 

 tentatively in the Jurassic period. The Franciscan rocks rest 

 unconformably upon the greatly eroded surface of the crystal- 

 line basement complex of granites, schists, and marbles, and 

 over the Franciscan lies the younger Cretaceous. The earliest 



