740 EDWARD H. NUTTER, WILLIAM B. BARBER 



Healdsburg. — At Healdsburg the schist area is nearly a mile 

 wide and more than four miles long. The thickness of the schist 

 is several hundred feet, though no exact measurements have 

 been made. From about half a mile south of the Junction 

 Schoolhouse, which is two miles southwest of Healdsburg, the 

 schists form a range of hills which extend in a general north- 

 westerly direction for several miles. In the southern end of this 

 area the schists are in contact with serpentine, and overlie a 

 boss or laccolite of it. The contact is clear and unmistakable, 

 as the rocks stand up above the soil, and hand specimens may 

 be secured which show the parting between the schist and ser- 

 pentine in a single fragment. In the serpentine boss is an 

 irregular mass of gabbro — possibly a result of magmatic differ- 

 entiation. In addition there are at least three serpentinized 

 dikes in the schist area northwest of the Junction Schoolhouse, 

 and also a small outcrop of diabase. 



With the exception of one place where it grades into shale, 

 the schist is entirely crystalline, and is composed mainly of 

 glaucophane, actinolite, garnet, epidote, and various light-colored 

 micas. Some layers are very quartzose and are composed 

 mainly of quartz, glaucophane, garnet, epidote and a little white 

 mica. The schists vary much in texture and mineral composi- 

 tion, but are easily recognized, as there are no other rocks like 

 them in this locality. 



South of the Junction Schoolhouse a fragment of actinolite 

 schist was found in the serpentine, and its plane of schistosity 

 makes a large angle with the planes of neighboring masses of 

 schist. It is clearly an inclusion and points definitely to the 

 serpentine being intrusive in and younger than the schist. 

 Ransome, in his paper on the geology of Angel Island, men- 

 tions similar inclusions in the serpentine there. 1 



About a quarter of a mile southwest of the Junction School- 

 house the schists appear to be overlain unconformably by Golden 

 Gate or Franciscan 2 sandstones. This relation is not entirely 

 x Loc. cit., p. 225. 



2 As the Franciscan or Golden Gate rocks are unfossiliferous, their identification 

 away from the type localities is based on lithologic features and field relations, and is 

 consequently uncertain. 



