PEARLS AND PEARL FISHERIES. 
405 
be sandy or gravelly and. the water clear. Other species, however, occur on muddy or 
clayey bottoms, where the current is slower. The references to rock bottom do not 
concern so much the immediate surface where the shells are found as the underlying 
bed on which the softer materials rest. In the matter of depth, also, the large pre- 
ponderance of answers in favor of shallow streams may mean not so much that the 
Unios greatly prefer shallow water as that they are more readily found and gathered 
there. The frequent allusions to “hard” or calcareous water tend to confirm the 
general impression that streams of this kind are favorable to the development of 
molluscan shells in both size and abundance. 
A few references to peculiar conditions may be noted, e. </., the Florida paper 
states that the best Unio growth is found in lakes with outlets, the water pure and 
fresh, but adds that it is sometimes sulphurous. One Texas paper (Colorado, Concho, 
San Saba, and Llano rivers) refers to the water as becoming slightly alkaline in dry 
times, and another (Colorado and Llano) makes a similar statement. A New York 
paper (De Grasse River and Plum Brook) mentions the water as brown or black — the 
clear, brown water of hemlock districts, familiar in northern New York. Iowa and 
Indiana papers state that spring-fed streams seem to be most productive of TJnios, and 
a New York account, describing them as found in rapid, gravelly streams, over lime- 
stone rock, adds that they are most abundant where the country has been cleared, “as 
the water is apt to be harder there.” 
The second inquiry, as to the geological character of the district, its rock, soil, 
etc., has been answered in 95 papers, more or less fully, though some refer only to the 
nature of the soil, or are otherwise incomplete. Of course no very exact scientific 
accounts could be looked for in such a body of responses, and the statements given 
are, for the most part, of a general character, though some are more detailed, and a 
few specify the geological horizon of the rocks at the localities described. Dividing 
the answers into two sets, one for the country rock and the other for the soil, they 
may be summed up as follows: 
Country rode : Limestone, 69; sandstone, 21; slate (and shale), 9; “flint” (or 
chert), 7. A few others are mentioned, viz: The Florida paper reports only “sand 
overlying clay or hardpan” (sand cemented by iron oxide), and Mississippi “only 
sand, gravel, and mud; no rock.” New York reports “iron-ore”; Pennsylvania, “coal,” 
and Texas “limestone and granite.” In many cases two or three of the rocks above 
named are mentioned in the same paper as associated in the region. 
Soil: Sand, 34; clay, 19; loam, 10; and a few other designations, as “mixed,” 1; 
“black,” 8; “calcareous,” 2, etc. In many cases two or three of these kinds are named 
together, as “sandy and clayey,” “ sandy loam,” and the like. 
The inference from these data is closely corroborative of that from the first 
inquiry — that a limestone region is favorable to the development of Unios. The nature 
of the soil seems to be of little or no importance in relation to the shells, as compared 
with the underlying country rock, outcropping or exposed in the river bottoms or 
along the bluffs. An Iowa paper remarks that “the presence of lime gives greater 
luster to the pearls,” and several allusions point to a general impression as to the 
advantageous character of a calcareous region. 
The principal geological references are the following : An Arkansas paper specifies 
the rocks of the district as “the magnesian limestone of the Lower Silurian, and 
encrinal marble and chert (Devonian); soil calcareous, with more or less sand.” The 
Ohio paper gives limestone and some shales, of Niagara, Clinton, and Cincinnati age 
(Silurian). One Tennessee circular refers to the Lebanon group (Lower Silurian) and 
