EXPLANATION OF THE PLATES. 



P L A T E 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 tliose 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 

 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 circums.tances 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 tiif whole vacuity, according to tlie length the process had been 

 carried; both of which varieties are well known to mineralogists. 



l-'ig. 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 laminae 

 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 stalagmile 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. 



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. Tlie cavity then remaining 

 has been filled by quartz from a change of character in the percolating 

 solution. 



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. 



Fio- 



JP/g 



