1890.] MICKOSCOPICAL JOUKNAL. 37 



fasciola^ and several other small varieties I cannot name. P. angula- 

 tu7n appears to be absent from Boston harbor, w^hile it is plentiful on 

 the Connecticut shore. 



But the variety that most surprised me was a species I supposed 

 foreign to our northeast shores. After washing off the lighter diatoms I 

 took a drop containing sand, coal-dust, &c., and examined it with low 

 power and counted twenty-seven entire frustules of Isthnila nervosa 

 in one drop. I had previously found fragments and sometimes a valve 

 in mv Connecticut shore gatherings, but supposed them accidental, 

 broughtfrom long distances by storms. In cleaning California specimens 

 I have found the valves separate easily in the acid, but these, although 

 much more severely treated, remained nearly all entire. This harbor- 

 mud contains quantities of cinders, coal-dust, and ashes from the steam- 

 ers and is hard to clean. I would like to hear from collectors as to 

 whether Istkmia nervosa is found living on our Atlantic shores. 



Marl from Fort Washington, Md.* 



By Prof. RICHARD FOSTER, 



WASHINGTON, D. C. 



The Eocene Tertiary formation crops out ten miles below the city 

 of Washington, at Fort Washington, on the banks of the Potomac. 

 This formation runs about parallel with the coast, and is seen on the 

 surface again in New Jersey, and there contains the noted marl beds 

 of that State. Marl is found at many localities in this formation, and 

 at different places it differs considerably in composition. That which 

 I have had tested from Fort Washington contains large quantities of 

 carbonate of lime, phosphate of lime, and sand, and traces of potash, 

 iron, and magnesia. In i860, Philip T. Tyson, then chemist of Mary- 

 land, made several assays of this marl with the following results. From 

 a specimen obtained from the southern corner of the District of Co- 

 lumbia, he obtained : carbonate of lime, 26.5 ; carbonate of magnesia, 

 2.7; silica, 42.4; silicate of alumina and iron, 25.4; potash, 20.0; 

 water, i.o. 



From a sample from Port Tobacco, he obtained the following : silica, 

 55.73 ; protoxide of iron, 9.45 ; alumina, 5.45 ; carbonate of lime, 

 12.92; potash, 2.07; water, 9.90. 



From samples taken near Fort Washington, the following analysis 

 was made : sand, 75.72 ; organic matter, 1.79 ; carbonate of lime, 5.32 ; 

 phosphate of iron, .44; alumina and oxide of iron, 14.77; potash, 



2.53- 



From a large number of assays of the New Jersey marl compared 



with a number of assays of Fort Washington marl, I obtain the follow- 

 ing comparison : 



A'e7u Jersey Marls. Fort IVashitif^ton Marls. 



Silica 50 50 to 75 



Alumina 6 to 7 5 " 14.44 



Oxide of iron 21 " 22 9 " 14 



Potash 9 " 14 traces " 3 



Carbonate of lime i " 3 5 " 27 



Water, etc 7 " 9 2 " 9 



* Read before the Washington Microscopical Society, Feb. 12, i88g. 



