1918] Davis: The Badiolarian Cherts of the Franciscan Group 273 



margins of these blocks the chert begins to take on a greenish color, 

 which gradually becomes more pronounced as it is traced outward 

 until it is impossible to tell where the chert ceases and the basalt be- 

 gins. There is apparently a perfect gradation from one rock into the 

 other. At Hunter 's Point, near a contact of chert with basalt, certain 

 of the inclusions seem to have progressed further than this. All trace 

 of original chert is lost, save for the presence in the rock of rounded 

 masses which resist erosion and appear to be more siliceous than the 

 rest of the rock around them. 



Condition op Cherts at Time of Intrusion 

 While the shale partings are often eliminated at igneous contacts, 

 there are instances in which the cherts and shales have not been 

 greatly changed by inclusion in the igneous rock. Occasionally the 

 igneous rock contains numerous streaks of red material, which in their 

 thicker portions, are found to be red shale exactly like that which 

 forms the shale partings. 



In places the pillow basalt is found to contain small blocks of 

 chert and shale which have been only slightly altered and in which 

 the shale partings have been preserved. Such small blocks prove that 

 in these places the basalts must have intruded the cherts after the 

 latter had consolidated. The intrusion of igneous rocks into uncon- 

 solidated muds and oozes would have caused a smearing out of the 

 bedding and obliterated it in these small masses. 



Mammillated Chert 



Aleso Creek, which drains into the Santa Ynez River from the 

 north, exposes a section of Franciscan gabbro, intrusive into radio- 

 larian cherts. In this gabbro there are great numbers of inclusions 

 of chert, many of which show mammilated surfaces very similar to 

 the mammilations observable on the surface of the chalcedonic lining 

 of cavities (plate 32). The blocks show no sign of shale parting, but 

 are massive. Many are prevailingly gray or green in color, but some 

 are pink or brownish red. The surfaces on which the mammilations 

 occur are not planes, but are warped surfaces of rather irregular form. 



The larger mammilations are elliptical in plan while the smaller 

 ones are nearly circular, but where they are numerous and closely 

 spaced the outlines of one mammilation interfere with those of an- 

 other and the resulting forms are polygonal. The diameters of these 



