482 PHYSICAL GEOGRAPHY. 



hunger. They have likewise very frequently been employed, under the 

 name of "tripoli," as a polishing material, and are excellent for that pur- 

 pose. In some parts of this country they go by the name of "marl," but 

 they are not examples of that substance, which is calcareous, being made 

 up of the remains of the shells of mollusca. Specimens from every 

 locality are desirable. 



Muds and Deposits from the bottoms of harbors, bays, lakes, ponds, 

 estuaries, and rivers. As a general thing these are not of very great 

 value to the microscopist for the rernains they contain, and it is only de- 

 sirable to collect them in localities or under circumstances where other 

 gathering cannot be made, or when they are known to contain any organ- 

 isms of great beauty or rarity. The blacker and softer the mud the 

 better, for, if it contains much sand or gravel, the minute organisms will 

 be present in just so much less proportion. As much as can be conven- 

 iently transported, say about a handful, should be collected, and, if possi- 

 ble, not dried, but placed in a bottle and tightly corked ; or, it may have a 

 little glycerine added to it, which will prevent its drying, — ^for it has been 

 found that muds, and especially those from salt-water, when once dried, 

 are only with difficulty broken down again so as to be cleaned. The 

 mud and slime attached to anchors, buoys, and submerged woodwork, 

 together with the scrapings from the bottoms of vessels containing shells, 

 plants, zoophytes, etc., may be simply dried in the sun, and then have a 

 label attached. The mud from beneath fresh water is of little value, as 

 it rarely contains any organisms of beauty ; but the marine forms found 

 in mud are occasionally fine, beautiful, and rare. 



Guano. This substance often contains species of diatomacese not 

 otherwise obtainable. It is the ammoniacal guanos alone, however, which 

 I have found to yield any great number of diatomaceous forms; but 

 there are certain guanos, of which one known as "Bolivian guano" is an 

 example, partly ammoniacal and partly phosphatic, which contain some 

 forms not otherwise obtainable. Quantities of a pound or two in weight 

 should be secured, and the exact locality of the island or other place from 

 which it was obtained, together with the latitude and longitude, and other 

 information that may be collected and deemed of interest, should be 

 marked in ink upon the label. 



Shell Cleanings. The sand, mud, algae, zoophytes, and similar matters 



