Dr. MacCulloch on a Vein in Limestone. 395 



What the situation of the present vein is with respect to the 

 horizon I have no means of knowing, nor is it a matter of much 

 moment, as it is tolerably evident that the strata which are now 

 visible on the surface of the earth do not always lie in the posi- 

 tions in which they were formed. There can be no question 

 that slides of this nature are of different scras, as we may often ob- 

 serve a succession of them in which the first has been shifted by a 

 subsequent one. Whether those represented in the present draw- 

 ings are referable to one or to different periods, there is no appear- 

 ance to decide, though their uniformity would lead us to suppose 

 that they were all produced by a single cause and at one time. 



Some unnecessary doubts appear to have been entertained by geolo- 

 gists respecting the formation of those smaller veins which have their 

 origin and termination in the rock where they are found, and which 

 have no communication from without. While one party has de- 

 nied their posterior origin to the rock in which they are contained, 

 and asserted that they were of " contemporaneous formation" with 

 the containing parts, another has had recourse to an igneous hypo- 

 thesis for the purpose of solving a difficulty of which the explana- 

 tion appears abundantly simple. It is universally known that many 

 rocks contain much water in a state of intimate mixture, or perhaps 

 combination, which they are subject to lose on drying or by expo- 

 sure to the air. From this cause they contract and form fissures. 

 Similar fissures occur from the ordinary subsidences and fractures 

 of parts either ill supported or subjected to external violence. Such 

 cavities being formed the process of infiltration commences. The 

 water existing in the rock percolates into the cavities, sometimes 

 forming crystals, and sometimes filling the cavity with a solid mass 

 of the matter which it held in solution. When silica exists in the 

 rock, veins of quartz are thus formed ; when lime, calcareous spar. 



