Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/97

 one case all the edges of the prism are replaced by planes, while in another, by the unequal truncation of three angles of a parallelopipedon, a pentagonal prism is produced. These prisms sometimes terminate in a pyramid, the faces of which correspond to the planes of the prism, and which is either complete or truncated: in other cases greater irregularities take place, but the extreme minuteness and transparency of the crystals frequently renders it impossible to ascertain their exact nature.

To compensate for the deficiency of large crystals of this substance, a profusion of that variety is to be found, which bears a general resemblance to amianthus, and it is popularly known in the country by the name of cotton stone. These filaments occupy the cavities of the trap, and are sometimes accompanied by analcime, as has already been remarked. They vary much in minuteness and delicacy, as well as in their state of aggregation, and hence many variations in their external aspect may be observed. At times they are placed in distinct straight needles, in other cases they are crowded into a dense mass, while in a third they are so entangled as to resemble a lock of cotton wool. Frequently they have the lustre of common silk, with its apparent dimensions, while they are in some instances so far attenuated as to resemble the silk of certain spiders. When the trap has fallen into powder, they are occasionally detached in light compacted balls, which are blown away by the winds and float on the surface of the water: in all these cases the microscope discovers their glassy transparency, but its powers are insufficient to determine their form, from the dazzling play of reflected and refracted light which they exhibit. In some rare instances this variety seems as if it passed into the mealy; in reality it becomes opaque and puts on to the naked eye a mealy aspect, which is however readily distinguished by the lens, from