Ilesozoic and Ccenozoic Geology and Paleontology. 151 



situations wiiere tiie force abates or is destroyed. The Salt Marsh 

 alluvium, which results from the deca}^ of salt marsh plants; the silt 

 brought over the mai'sh by the tides; and from the alluvial soil 

 brought down by streams which empty through these marshes. He 

 mentioned the submarine forests on the coast, and on Martha's Vine- 

 yard, and numerous deposits of peat, and the processes by which it is 

 produced. He observed how rapidl}' the New Red Sandstone disinte- 

 grates and unites with the soil, giving a decidedly red hue to extensive 

 tracts of laud; and likewise the gneiss, which is found disintegrated 

 to a depth of from six to_ten feet, and thus covers the eartli and ob- 

 scures the rocks even in the hillj^ districts.. Some varieties of trap, 

 sienite, mica, talcose aud argillaceous slates are similarly affected, and 

 even quartz-rock is shown to slowly decompose by the action of the 

 weather. As evidencing the latter fact, it is mentioned that the; name 

 of John Gilpin had been painted upon a smooth boulder of granular 

 quartz within the past 150 years, and that the paint had so protected 

 the surface beneath it, while the decomposing process went on over 

 other parts of the rock, that the name is now found perceptibly ele- 

 vated on rubbing the fingers over the stone. Three causes — rains, 

 frost and gravity — are said to be constantly operating to degrade the 

 hills and the mountains. In precipitous trap-ridges, water penetrates 

 fissures, freezes, and breaks asunder the masses which constitute the 

 slopes of broken fragments or debris of rocks, which arrest the atten- 

 tion on the mural faces of the greenstone ridges in the Connecticut 

 valle}'. The gneiss rock, in Worcester count}^, abounds with sulphuret 

 of iron, which is continually undergoing decomposition by the action 

 of heat, air and moisture, and becoming changed into an oxide and 

 sulphate. The oxide imbibes carbonic acid from the atmosphere, and 

 is changed into a carbonate which is soluble in water; or this oxide is 

 washed into cavities, where it meets with water containing carbonic 

 acid, by which it is dissolved. Once dissolved, it is transported to 

 ponds and swamps, where it is deposited by evaporation, and forms 

 the well known bog iron oi'e. Rocks containing manganese are like- 

 wise undergoing decomposition, and producing, in a similar manner, the 

 oxide of manganese. 



The ridge of bowlders on the margin of some ponds, where the bot- 

 tom is free from them to a considerable extent, is accounted for by the 

 expansion of the ice in lifting them from the bottom and crowding 

 them out, while there is no force on the melting of the ice to draw 

 them back. 



