ABSTRACTS AND DISCUSSIONS OF PArERS 401 



breccia, consisting of amphibolite, that is probably a metamorpliosed diorite, 

 inclosed in a matrix of very similar character. 



In the Alma district, in the Mosquito Range, an intrusive mass of diorite 

 occurs imbedded in a matrix that is almost identical with the fragments. 



In attempting to describe these occurrences, in which the structures desig- 

 nated as flow-breccias are a characteristic feature, the writer has felt the need 

 of a descriptive term that may l>e used without reference to the exact nature 

 of the inclosed fragments of the enveloping fluidal matrix, and would venture 

 to suggest the term rhyoclastic as suitable for this purpose. 



A rhyoclastic rock, then, would be an igneous rock in which occur numerous 

 inclusions of fragments of similar or of different material, so as to resemble 

 more or less closely a true breccia. 



Unfortunately, the term flow-breccia has been employed by Iddings in a 

 quite different sense from that originally given by Cross. In his book on 

 Igneous Rocks. Iddings" writes as follows : "When exploded fragments of 

 molten magma, large or small, fall together in a still heated condition, as may 

 readily happen within the crater of a volcano or the mouth of a fissure, they 

 may be plastic enough to weld together in a more or less compact, coherent 

 mass. This may subsequently flow like other lava, and is known as flow- 

 breccia." 



While such a rock may, of course, very properly be called a flow-breccia, it 

 does not seem to he wise or in conformity with well established custom to 

 change the definition as originally given by Cross. And yet it would be well 

 to recognize the structure to which Iddings has applied this term. This could 

 be done and confusion avoided by designating such rocks as welded flow- 

 breccias. 



We have, then, two types of flow-breccias : a rhyoclastic flow-breccia, where 

 foreign fragments are caught up in great abundance in a flowing igneous 

 matrix, and a welded flow-breccia, where fragments of a partially plastic 

 igneous magma are shattered by some explosive act and flow together again 

 b.\- a process of welding. 



Presented in full extemporaneously. 



Discussion 



Prof. Erasmus Hawortu expressed a desire for further information on flow- 

 breccias and as to how this breccia is distinguished from other types of breccia. 



OEOLOOY OP A PORTION OF THE SANTA YNEZ RIVER DISTRICT, SANTA 



BARBARA COUyTV, CAIJFORXJA 



(Ahstrarf) 



Tlic region under discussion cniliraccs the Santa Ync/. Mountains east of the 

 San Marcos I'ass, the Santa Ynez River Canyon, and the mountains lying im- 



" J. V. IddliiKs : iKticons Kocks, vol. iV V.to;), p. ;;.;i. 

 *^ Introduced by A. C. Lawson. 



