1880.J NATURAL SCIENCES OP PHILADELPHIA. 345 



roots protectino- the rock, and retaining the earthy matter would 

 longer hold its own. In the former case with the gradual washing 

 away of the earth the larger trees will have to find a lower 

 level ; the summit condensing more moisture, and having a cooler 

 atmosphere, would form heavier masses of longer enduring snow, 

 and thus keep down from tall growth the younger trees left as the 

 older and larger ones retired. They would have to be low bushes 

 by the absence of earth for vigorous groAvth, and remain trailing 

 bushes, through the superincumbent and long continued mass of 

 snow. 



We thus see that though a long continued mass of snow has 

 much to do in marking a timber line, that line is precedent to the 

 snowy mass. The primar}^ cause is the gravitation of disinte- 

 grated rock— tlie movement of the hill top towards the sea. From 

 the moment the mountain reaches its highest point it commences 

 its downward march. The entire reduction of the highest to a 

 level with the plain is but a question of time. The frost and rain 

 and melting snow will do it all, and this reduction, bringing down 

 not only the earth, but cold-loving plants to warmer levels, must 

 continually change the aspects of vegetation, as well as perpetually 

 vary the timber line. 



In low hills as well as in high mountains the forces of gravita- 

 tion are also at work. But the sides are seldom so steep as in the 

 loftier hills, — the rains do not gather with such force nor are 

 the melting snows of near the same duration. There are sudden 

 washes, but not the continuous roll of the earth to the bottom. 

 In time they maj^ exhibit the same phenomena of the disappearance 

 of species from their summits as their loftier brethren ; but the 

 centuries here will gather much more slowly to produce a similar 

 effect. 



In conclusion he would say briefly that the " timber line " of 

 high mountain tops results from the washing down of the earth 

 from the higher elevations. 



Mr. Redfield remarked that there could be no doubt that 

 influences other than climatic (such for instance as the washing 

 away of soil, mentioned by Mr. Meehan) do often modify and 

 change the timber-line upon mountains. But he was unable to 

 accept Mr. Meehan 's views as to the insignificant part played by 

 climatic causes, and still held them to be the prevailing factor in 

 the problem. Dr. Parry's explanation by the weight and depth of 

 winter snows might not always be the correct one, but snow and 

 ice must be very important agents, and Mr. R. thought that in 

 considering climate, we should have regard not merely to the 

 present period, but to past great secular periods. He then referred 

 to the glacial age, when not only the White Mountains, but all 

 New England was capped with a vast ice-sheet, which under secular 

 changes gradually retreated, leaving only the mountain tops 

 covered. The slow retreat of the glacial covering was followed 



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