190 Bog-Trotting for OrcKids 



deeper, layers of rock will be cracked and broken, until 

 finally the pot-hole formation will be destroj^ed. The 

 upper or third basin is located in the harder portion of 

 the lime and marble bed-rock, portions of the marble 

 being highly polished. The marble brook-bed glittered 

 in the noonday sunshine. Pot-holes are formed origin- 

 ally by a boulder, which — carried in the currents of a 

 stream — lodges in a depression of the bed-rock. It 

 bores gradually into its resting place, until, in the 

 course of ages, it has worn the walls of its basin into a 

 deep hollow^ at the same time wearing itself away, at 

 last being carried off as a pebble. It may be that these 

 holes are sometimes formed in a slightly different way. 

 Dimples and fissures often occur in rocks, and if the 

 water and pebbles circle about' these cracks they prob- 

 ably eat down through the soft layers of rock, and thus 

 loosen a revolving stone from the bed-rock itself. It 

 would then fit the pot-hole closely for ages, revolving 

 as the currents become forceful in freshets. The pot- 

 holes along the granite ridges in Bronx Park, New 

 York City, as well as on the Canaan Hills — nearly one 

 thousand feet above the Merrimac and Connecticut 

 river-beds, — reveal the erstwhile revolving stones now 

 motionless in their basins. 



After remaining in the region of the Wash-Tubs an 

 hour or more, I followed down the lateral moraine or 

 wooded ridge along the stream, w-hich became rocky in 

 the heart of the hemlocks. Upon a broad table-like 

 rock, I found a large mat of Walking Ferns. It ap- 

 peared about four feet square, and contained the most 



