274 University of California Publications in Geology [Vol. ll 



mammilations range from one-half inch up to four inches. The usual 

 diameter is from one inch to one and one-quarter inch. They are 

 rather flat in profile. In examining blocks of chert which show these 

 mammilated surfaces it is often found that, by proper application of 

 the hammer, it is possible to develop still other mammilated surfaces 

 beneath the outer one. The inner mammilations have no correspond- 

 ence with those on the outer surfaces, having different radii and dif- 

 ferent centers. 



A question arises as to the origin of these peculiar cherts. It might 

 be urged, perhaps, that they are not radiolarian cherts at all, but are 

 the result of hydrothermal deposition of silica in the gabbro. This 

 hypothesis is unwarranted for the reason that the masses do not occur 

 in veins nor do they have any connection with veins. They are iso- 

 lated blocks often with rather angular outlines. They are unques- 

 tionably derived by alteration of the ordinary type of radiolarian 

 chert, since they are reddish in color, have the texture of chert and 

 are in all respects similar to many varieties of metamorphosed chert 

 whose origin is definitely known to be due to thermal metamorphism 

 at igneous contacts. Moreover, thin sections of some of them have 

 revealed radiolaria. 



The mammilations might be due to the recrystallization of the 

 chert under the action of heat, resulting in the development of a 

 spherulitic structure on a scale larger than is commonly found. How- 

 ever, thin sections of these cherts show a cryptocrystalline aggregate 

 of silica with no fibrous chalcedony. 



Another possibility is that the surfaces are the result of incipent 

 fusion. Certain specimens of chert from Marin County show a round- 

 ing probably due to fusion. These mammilated cherts from Aleso 

 Cation do not appear to be of this type, since the rounding is not con- 

 fined to the surface of the blocks. Additional surfaces may be devel- 

 oped beneath the outer one. 



It would seem that the peculiar surfaces are the result of the 

 spalling off of the chert governed by a system of irregular, curving 

 fracture-cracks. Systems of curving cracks are often developed in 

 chert masses under the action of heat, and something of the sort 

 seems to have occurred here. This idea is supported by the finding 

 of numerous lenslike quartz veinlets within the mass of the rock. 

 These are not straight but curve more or less parallel to the mamil- 

 lated surfaces. 



