W. Upham — Marine Shells in the Boston Till. 359 



Algous Pulverulent 



Geyserite. sinter ? deposit. 



Si0 2 .. 90-28 92-47 74-63 



A1 2 3 3-00 2-54 15-59 



CaO.. 0-44 0-79 1-00 



MgO trace 0-15 trace 



Na o 0-30 



K 2 6._. 1-02 



Ignition.. 6-24 3-99 7'43 



Total 99-96 99-94 99-97 



The first is typical geyserite, undoubtedly an evaporation de- 

 posit ; the second is the laminated sinter already alluded to. 

 The third is a pure white pulverulent deposit resembling a 

 block of diatomaceous earth, but composed of impalpable par- 

 ticles of glass. Its composition corresponds to that of rhyolite 

 (the rock from which the Rotorua waters also issue) with the 

 alkalies leached out and replaced by water. 



Summary. 



The study of the origin of the deposits of siliceous sinter 

 found in the Yellowstone proves that they are largely formed 

 by the vegetation of the hot spring waters. Waters too poor 

 in silica to form sinter deposits by any other cause may be ac- 

 companied by beds of siliceous sinter formed by plant life. The 

 extent and thickness of these deposits establishes the importance 

 of this form of life as a geological agent. 

 Washington, D. C, Jan. 20, 1889. 



Art. XXXYIIL — Marine Shells and Fragments of Shells 

 in the Till near Boston • by Warren Upham. 



[Read before the Boston Society of Natural History, Dec. 19, 1888.] 



The fossils here described, occurring in drift deposits near 

 Boston, and belonging wholly to species that are still living in 

 Massachusetts bay, have been previously noticed by several ob- 

 servers, who have regarded them as evidence of a marine 

 submergence within the Pleistocene or Quaternary period. 

 Instead of this, my observations made during the past summer 

 and autumn show that these fossils were transported from the 

 bed of the sea on the north by the ice-sheet in the same man- 

 ner as the materials of the drift, including its bowlders and 

 rock fragments, large and small, have been carried various dis- 

 tances from north to south, being often deposited at higher 

 elevations than the localities from which they were brought. 



