Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/492



PLATE 10.

Agate pebbles from the hill of Kinnoul in Perthshire.

Fig. 1. Represents a hollow nodule containing small chalcedonic stalactites. It is placed in the same position which it appears to have occupied in the rock where it was formed. On considering its construction and comparing it with those of stalactitic caverns on a large scale, it will appear probable that after the deposition of siliceous matter which now forms the exterior crust had taken place, the process of infiltration became limited to its upper I part. Thus the superior pendents were formed, while the dropping of the chalcedonic solution from their points has produced the corresponding stalagmite below. Where the infiltration has been most easy the stalactite and stalagmite have met, while the total suspension of the process in another part, has left a portion of the cavity unoccupied. It is easy to comprehend how such a nodule might be found filled with water, or how it might be occupied with quartz crystals instead of chalcedony. It is equally easy to see, that it might under certain circumstances give access to a solution of carbonated lime, in which case the interior would be occupied by a calcareous crystal; a circumstance extremely common. The crystal in such a case would either be found independent within the cavity, or filling up me whole vacuity, according to the length the process had been carried; both of which varieties are well known to mineralogists.

Fig. 2. This example presents a variety of the same process very common in the chalcedonic nodules of Faroe. The stalactite in this example is tortuous, and the bottom of the cavity is filled with horizontal layers of the same substance. Where these specimens are found to consist of parallel laminæ perforated by stalactitic forms, whether straight or crooked, they present a very mysterious aspect, but their formation is easily explained in the same way. The stalagmite in this case assumes the same diffused flat form that calcareous ones often do in large caverns, while as it continues gradually to rise it surrounds and entangles the dependent bodies without losing its parallelism, until the whole cavity is filled and consolidated into one mass.

Fig. 3. This figure represents one of the more obscure cases that occur in the chalcedonic nodules of trap. It may perhaps be explained by supposing that the straight parallel layers were first formed till one half of the cavity was filled, and that the layers parallel to the cavity, which appear above, had been deposited afterwards by the more tedious process to which the ordinary concentric nodules owe their existence. The cavity then remaining has been filled by quartz from a change of character in the percolating solution.

Fig. 4. The same process appears to have been carried on in this specimen, with the variation only that the whole of the upper and last remaining cavity has been filled by the concentric layers.