82 proceedings: geological society 



J. S. Diller: Was the new lava from Lassen Peak viscous at the time 

 of its eruption? That it was viscous is indicated by the following 

 considerations: 



1. The material that rose from the volcanic chimney into the crater 

 on the summit of Lassen Peak spread in all directions and filled the fun- 

 nel-shaped crater to its rim, forming a lid to the volcano. 



2. Upon reaching the crest of the crater's rim the lava overflowed at 

 the lowest points, one stream flowing down the western slope of the 

 peak and another toward Lost Creek on the northeast. 



3. The flow down the west slope is about 1000 feet in length and is a 

 normal stream of siliceous lava with very rough broken surface. That 

 it was a hot, viscous mass beneath the surface at the time of its eruption 

 is proved by the written testimony of a number of observers, who from 

 9.30 to 11.30 on the night of May 19, 1915, watched the glow and 

 flashes of light from the flowing, breaking lava as, in plain view, it 

 crossed the crater rim and descended the slope. 



4. In the overflow stream at the head of Lost Creek the delicate, 

 steam-torn lava was bent and folded, as if viscous, while advancing across 

 the crater rim, and much of it was carried away by the tremendous 

 blast of hot gas (from beneath the lid) that devastated the Hat Creek 

 country. 



5. When the hot blast escaped from beneath the lava lid a portion of 

 the lid, apparently where hottest, subsided into the former crater and 

 exposed a number of fracture surfaces of lava blocks. On some of these 

 surfaces there are distinct lines of viscous flow, and as the surfaces origi- 

 nated dining the subsidence following the great eruption in May, 1915, 

 the lava must have been viscous at that time. 



G. W. Stose: Age of certain shales in Cumberland-Lebanon Valley, 

 Pennsylvania. The shales referred to are in detached areas in the lime- 

 stone valley. The limestones range in age from Lower Cambrian to 

 Ordovician and may be divided into seven formations. The shales 

 were regarded by the Pennsylvania Geological Survey as equivalent 

 to the shales formerly called Hudson River but now called Martinsburg, 

 which overlie the limestones and adjoin them on the northwest. In 

 weathered outcrop the various shales are closely similar and, as east of 

 Harrisburg they have been brought together like parts of the same for- 

 mation, this miscorrelation is not surprising. 



The real age of the shales in the detached areas was determined only 

 after tracing the seven formations of the limestone from their known 

 outcrops in the Carlisle quadrangle, southwest of Harrisburg. The 

 largest area of the shales south of Harrisburg was proved, by its rela- 

 tion to the Elbrook limestone, to be the Waynesboro shale of Middle 

 Cambrian age, and typical purple siliceous shale of the Waynesboro 

 was later found in it. The irregular relations to other formations on 

 its north side are due to overthrust faulting, which terminates the area 

 against the Martinsburg shale east of Harrisburg. The Waynesboro 

 shale occurs also in prominent hills east of Schaefferstown and in small 

 hills near Shillington, southwest of Reading. The shale thins abruptly 



