530 GEORGE DAVIS LOUDERBACK 



basic soda-lime feldspars with magnetite, etc. This is varied, how- 

 ever, by the partial or even complete substitution of more or less 

 hornblende for the pyroxene, or by the more or less complete dis- 

 appearance of the feldspar. Alteration is generally very marked, 

 chlorite, calcite, limonite, and kaolin, being common products of the 

 changes; and the rocks are frequently crushed, sheared, slicken- 

 sided, and traversed by calcite veinlets. 



The exact relationship of these rocks to the Dillard series can, 

 in the majority of cases, not be determined, and some of the bodies 

 may represent contemporaneous lava flows. Proof of this was not 

 found in any individual case, but i-n many areas the fact of intrusion 

 into the sandstones, and especially the cherts, with small apophyses 

 breaking through and spreading between the layers, is perfectly 

 evident. All of these rocks may therefore be intrusive. Some, 

 however, apparently have an original clastic structure and may be 

 tuffs. They are called basalts in the text of the Port Orford folio, 

 but as they vary so much in texture and mineral composition, and as 

 their textures, mineral composition, and exact geological relations are 

 frequently indeterminable in the field, and, finally, as the almost 

 uniform green coloration, especially on a fracture surface, is so 

 universal, the writer has used the comprehensive field term "green- 

 stone" as more expressive of these characters and variations. 



The feldspathic granular rocks. — The rocks discussed under this 

 broad title include those described as " metagabbros " in the text 

 of the Roseburg folio (except such as have been described above as 

 greenstones), and the "gabbros" of the Port Orford quadrangle. 

 This is evidently a genetically heterogeneous group, which the brief- 

 ness of the writer's study has left in a very unsatisfactory state. We 

 may distinguish, however: 



a) A group of basic gabbros which are genetically related to the 

 serpentines (altered peridotites, to be described later). These rocks 

 are generally essentially coarse granular holocrystalline aggregates 

 of diallage and a very basic soda-lime feldspar, with various acces- 

 sory minerals, and from their close association with the serpentines 

 in the field, and the habit of their component minerals and the inter- 

 mediate forms which exist, they have presumably been derived from 

 the same magma which gave rise to the serpentines. 



