254 FIN LAY 



(grossularite) and vesuvianite. These are found by the score 

 over the whole region. They offer most interesting problems 

 in contact metamorphism. The main mass of the limestone 

 gives evidence of the dynamic effects of laccolithic intrusion. 

 The siliceous bands, which appear as dense flinty included 

 masses half an inch thick and a foot or more in length may 

 often be seen to have been broken and faulted repeatedly. 

 These effects were not observed at a distance from the andesite. 

 It is impossible to say how thick the cover over the laccolith 

 was. It is believed that the igneous rock was viscous rather 

 than liquid. The amount of silica present, sufficient to allow 

 for the formation of free quartz, is evidence in favor of this view. 

 The laccolithic intrusion is of comparatively small extent with 

 moderately high doming. No evidence could be gathered 

 which would help decide the question as to whether or not the 

 incoming of the molten rock was aided by the relief from pres- 

 sure consequent to the initial stages of anticlinal folding. The 

 limestone at a short distance from San Jose lies nearly hori- 

 zontal, but the shaly member of the series constantly shows 

 pressure effects. Exposures of this rock are found to the east 

 of the Pic de Diablo, replacing the limestone in the foothills of 

 the San Carlos Mountains. It is here a drab or gray coarse 

 shale, thinly bedded, and everywhere broken by countless joint 

 planes. These give it a platy parting horizontally. The ver- 

 tical joints are frequently so numerous as to cut the shale up 

 into small pieces. These fragments two inches or more in di- 

 ameter are quickly broken out and rounded by the agencies of 

 weathering. 



In the slopes of Mt. Ladinas, and on Mt. Armadillos, as 

 well as various points to the southwest and southeast of it the 

 andesite has broken through the limestone cover in the form of 

 dikes. 



IV. STREAM ADJUSTMENT 



The phenomena of drainage pecuhar to eroded laccoliths 

 have recently been treated in a paper by Dr. T. A. Jaggar, Jr., 

 in the Twenty-first Annual Report of the U. S. Geological Sur- 



