404 proceedings: geological society 



largest gratings made. Mr. Anderson stated that all of the gratings 

 were made on speculum metal. This is crystalline and therefore, 

 owing to occasional pitting, subjects the tool to slight falls with the 

 result that the diamond may chip ; the natural edges of the diamond are 

 used for cutting; with care one tool will rule from 6 to 10 miles. The 

 diamond would stand indefinite use on soft metals such as silver; if 

 soft metals could be given as high a polish as speculum metal it would 

 be possible to rule large gratings. Messrs. Humphreys and Wright 

 cited a number of examples of diamonds that cannot be cut by the 

 lapidary; they are designated as "knots." Mr. Bauer asked what is 

 the largest number of lines ruled to the inch. Mr. Anderson stated that 

 he had himself ruled 15,000 lines to the inch and was building a machine 

 to rule 30,000; Professor Rowland had ruled one grating with 86,000 

 lines per inch but that grating had been lost; among the gratings ex- 

 hibited was one with 43,000 lines per inch by Professor Rowland. 



The chair extended to Messrs. Anderson and Hewlett the thanks of 

 the Society for their very interesting papers. 



J. A. Fleming, Secretary. 



THE GEOLOGICAL SOCIETY OF WASHINGTON 



The 308th meeting was held in the lecture room of the Cosmos Club 

 on April 12, 1916. 



REGULAR PROGRAM 



J. S. Diller: Geologic history of Lassen Peak. Lassen Peak is an 

 ancient volcano at the southern end of the Cascade Range and fills the 

 gap between the northern end of the Sierra Nevada and the Klamath 

 Mountains composed largely of old sedimentary rocks. 



Its volcanic activity began near the close of the Eocene, was great- 

 est during the Miocene and Pliocene and decreased in the Quaternary 

 to near extinction. 



Lassen Peak is a volcano of large type surrounded by many smaller 

 ones of later date, the whole being built up by many explosive and 

 effusive eruptions of a notable variety of lavas. The earliest lavas 

 are andesite, but the differentiated magma appeared later on the one 

 hand as dacite and rhyolite, and on the other hand as basalt and quartz 

 basalt. 



In developing the peak the great volcanic vent migrated nearly 4 

 miles to the northwest, erupting first andesite, then dacite, which built 

 up Lassen Peak to its present height, 10,460 feet. Later, only a few 

 centuries ago, dacite was erupted at the northwest base of the peak 

 forming Chaos Crags, but finally activity began again May 30, 1914, 

 erupting dacite in the old crater of the highest summit. The basalts 

 are mainly in the peripheral region. 



In the discussion Diller asserted that the flashes of light, cloud glows 

 and rocket-like incandescent bombs shot from the crater prove that at 

 least some of the new lava erupted was hot enough to be luminous, and 



