1904.] 



HAEHL AND ARNOLD — THE MIOCENE DIABASE. 



88 



of it may come from a weathering of the feldspars of the basaltic 

 fragments. Patches of isolated calcite are also found in most of 

 the slides of both the basaltic and diabasic rocks. Fig. 13 shows 

 a slide of diabase tuff from the hill east of the Langley ranch house. 

 A secondary calcite vein (C) and small isolated patches of calcite 

 (C) are seen in this slide. Veins of chalcedony and limestone 

 dikes or intrusions are also common in the tuffs. 



Limesio7ie Dikes. 



One of the most interesting phenomena met with in a study of 

 the Langley Hill-Mindego Hill igneous area is the occurrence of 

 limestone dikes or intrusions in the tuff beds. The best exposures 

 of these dikes are found in the ridge to the north of the Langley 

 ranch house. Figs. 14 and 15 show transverse and longitudinal sec- 

 tions of this ridge, respectively. Similar dikes occur in the tuff 



Fig. 14. North-south section through the Langley ranch, showing the strati- 

 graphic relations of the tuffs which contain the limestone dikes. 



which makes up the ridge running southeast from the top of Lang- 

 ley Hill, and also in the tuff exposed along the Searsville-La Honda 

 road north of La Honda. 



Fig. 15 shows the relative position and size of the principal dikes 

 exposed in the ridge north of the Langley ranch house. These 

 dikes are composed of a more or less pure limestone, in which are 

 generally imbedded fragments of the tuff of varying sizes. The 

 clastic origin of these dikes is shown by their gross structure, their 

 petrographical character and the occurrence of organic remains in 

 nearly all of them. The dikes vary in width from a fraction of an 



PROC. AMER. PHILGS. SOC. XLIII. 175. C. PRINTED MARCH 9, 1904. 



