DOME STRUCTUKE IN CONGLOMERATE 615 



Both sections adjourned for luncheon together, at the Manhattan 

 Square hotel, and reconvened as a body at 2.30 o'clock p m. 



. Acting President Davis occupied the chair and gave the floor to W. G. 

 Tight, who presented to the Society a most cordial invitation to hold its 

 next meeting at Albuquerque, New Mexico. Two other equally cordial 

 invitations were before the Council, but the Society by a large majority 

 expressed its approval of holding such a western meeting and recom- 

 mended the invitation to the Council for favorable consideration. 



The Acting President then read the posthumous presidential address 

 of Professor Israel C. Kussell. This address is entitled 



CONCENTRATION AS A GEOLOGICAL PRINCIPLE 



and has been printed as pages 1-28 of this volume. 



The Society then divided into two sections, as before, and proceeded 

 with the reading of papers. 



In the Physical and Structural section, under the chairmanship of the 

 Acting President, the first paper read was 



DOME STRUCTURE IN CONGLOMERATE 

 BT RAUPH ARNOLD 



[Abstraet] 



In Eagle Rock valley, near Pasadena, California, are several dome-sMaped 

 structures, developed in lower Miocene conglomerate, of water-worn plutonie 

 rocks. Tbe lairgest dome is Eagle rock, about 80 feet higli. Tbis, in a 

 general way, I'esembles the granite domes of the Sierras, but is much smaller. 

 Scales, or plates, of the rock from a fraction of an inch to as much as 8 feet 

 in thickness cover much of the sloping dome surface, while large quantities of 

 broken scales from detrital heaps at the base of the rock. Not far distant 

 from Eagle rock are other examples of the dome surface. In all cases the 

 scales are not found on slopes of less than 30 or 40 degrees nor on the exposed 

 southern faces or other portions of the rocks which are kept practically dry. 

 The cracks under the scales always penetrate upward appi'oximately parallel 

 to the outer surface. The incipient cracks are closely followed by a zone of 

 weathering which shows lamination parallel to the crack. Iron and other 

 stains emanate from the cracks. 



Two general theories have been advanced in explanation of such peculiai 

 structure, all, heretofore, however, based upon observations of the develop- 

 ment of domes in granite. According to one theory, the separation of the 

 granite into curved plates is an original structure, antedating the sculpture 

 of the country and determining the peculiarities of form. According to the 

 5ther theory, the structure originated subsequently to the form, and was 

 caused by some reaction from the surface. The conclusions reached by the 

 writer regarding the origin of the conglomerate domes are as follows : 



In the first place, they are locally hardened portions (gigantic concretions. 



