Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/234

 generally of a small size, and of a spheroidal or ellipsoidal form. Their fracture is crystalline, and they are often perfectly transparent, but in all cases where they are in immediate contact with the trap they possess no external crystallized form. It is only when in contact with quartz or agate that they exhibit an external as well as an internal appearance of crystallization. In other words, they appear to bear the impression of the basalt when they are in immediate contact with it, while on the contrary, when in contact with quartz or agate, they impress their forms on these. Calcareous spar thus assists in forming a compound nodule not uncommon in these rocks. The spar is sometimes crystallized at liberty in a cavity of quartz or agate; at other times it is closely invested by quartz, forming the center of a solid nodule, or is dispersed in small crystals irregularly scattered through the whole pebble. Both these substances occasionally retain their perfect characters, even when in immediate contact; while in other instances the quartz for some small distance in the vicinity of the calcareous crystal is converted into chert or into agate.

Quartz is found among these rocks in the usual variety which it exhibits when it is an inmate of trap. In its simplest form it is a crust investing a cavity, and terminating interiorly in assemblages of crystals of various sizes and of various colours, white, brown, and amethystine. The exterior quartzy crust often puts on the character of chert, sometimes that of chalcedony; not infrequently the chalcedony appears in the form of a stalactite, of which the several icicles are encrusted with beautiful and crowded assemblages of crystals. These stalactites of crystals depend, as is usual in stalactites, from the upper parts of the cavity. The nodules of this description are of various magnitudes and often of a foot or more in diameter, and, like the smaller ones, are shut in and