Page:Excursions (1863) Thoreau.djvu/77

Rh the sides of the ruts resembled exactly asbestos in the disposition of their needles. Around the roots of the stubble and flower-stalks, the frost was gathered into the form of irregular conical shells, or fairy rings. In some places the ice-crystals were lying upon granite rocks, directly over crystals of quartz, the frost-work of a longer night, crystals of a longer period, but to some eye unprejudiced by the short term of human life, melting as fast as the former.

In the Report on the Invertebrate Animals, this singular fact is recorded, which teaches us to put a new value on time and space. "The distribution of the marine shells is well worthy of notice as a geological fact. Cape Cod, the right arm of the Commonwealth, reaches out into the ocean, some fifty or sixty miles. It is nowhere many miles wide; but this narrow point of land has hitherto proved a barrier to the migrations of many species of Mollusca. Several genera and numerous species, which are separated by the intervention of only a few miles of land, are effectually prevented from mingling by the Cape, and do not pass from one side to the other Of the one hundred and ninety-seven marine species, eighty-three do not pass to the south shore, and fifty are not found on the north shore of the Cape."

That common muscle, the Unio complanatus,