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belehed forth a flood of lava that formed high ranges of mountains and 
laid the rocky floors of many valleys in its flow to the lower levels, 
and perhaps during this great upheaval the limestone bedrock of the 
Mississippi and adjacent valleys was pushed up to its present height, 
out of the sea, which its marine fossils indicate. 
At this time it is possible that part or all the continental moun- 
tain ranges were raised to their present elevations, probably higher, 
in which time and the adjustment of the earth’s crust to the new order 
of things after these upheavals, may have made some minor changes. 
With this general elevation of the land the barriers that held the 
waters of the great internal lake system of that time were broken, 
the water rapidly drained off and the rush of floods cut the river 
channels that form the drainage system of the continent, which the 
storms and floods subsequently have completed as we have it today. 
As the geographical range of land shells is a question of consid- 
erable importance to conchologists, | wish to call especial attention to 
the rivers and drainage system of the continent, as the principal 
avenue and means of distribution of these slow-moving creatures. 
Little study of the map of North America will show that all the 
principal rivers that form the drainage system of the continent have 
their souree in the district formed by, or near, the union of the states 
of Idaho, Montana, Wyoming, Utah and Nevada. These rivers flow 
to the Aretie oeean at the north, to the east and south, into the At- 
lantic, to the southwest, the west and northwest into the Pacifie ocean, 
and curious enough, this district, the source of all these rivers, is also 
the metroplis of all the genera of the universally distributed American 
land shells. TI have collected in this district myself, the following 
genera: Suecinea, Vitria, Tonites, Cireinaria, Helicodicus, Patula or 
Pyramidula, Polygyra, Vallonia, Pupilla, Cochlicopa, Polygyrella, 
Hemphillia and Prophysaon. 
The last three genera, however, are confined in their geographical 
range to the western slope of the continent so far as we know at the 
Present time. 
There is little doubt in my mind, that the universally distributed 
genera and species have been seattered over the continent prineipally 
by the drainage system. 
During heavy storms and great floods, whole colonies, as well as 
Individual shells, are washed into the streams with the woodland 
debris and carried far away from their native haunts in the higher 
areas of the mountain districts, to the lower levels of the valleys, 
Where lodgments are made, and colonies are formed, perhaps after 
Many failures, but in the course of time beeome permanently estab- 
lished and spread over the adjacent territory. 
