BY THE REV. W. B. CLARKE, M.A., F.G.S., &c. 



temperature of 212 degrees Fahrenheit, is, I believe, quite capable 

 of producing, in time, much of the changes observed. 



The more direct agency of heat will be dealt with when we 

 enter upon the subject of Trappean Transmutations. 



It must, however, be borne in mind, that although siliceous 

 rocks in contact with granite are transmuted, it is not always so. 



The influence in all cases must be in proportion to proximity ; 

 and there are instances in which granite does not appear to be the 

 direct cause at all, for it is of a totally different origin since it 

 occurs sometimes where no granite exists. 



In the case of our great Hawkesbury rocks, so common about 

 and under Sydney, the ground may be seen full of bright 

 sparkling crystalline particles, which it is difficult to understand 

 as belonging to a sandstone formed by drift and deposition alone, 

 unless we suppose, what is probably the case, that the original gran- 

 itic and quartz detritus, of which much of the rock is composed, 

 has, since its formation, undergone a transmutation which has 

 produced the crystalline particles and facets that stud the whole 

 of the gtrata, and betray no kind of abrasion whatever. They 

 may have been formed by secretion from a silicified menstruum in 

 which the deposits took place. 



I have washed away by water and acid the coloured and 

 cementing matter of most of our sandstones, and also of the red 

 sands of the interior desert, and have then found when under the 

 microscope, that the particles are generally only crystallised in 

 part, seldom assuming more than a rude resemblance to hexagonal 

 crystals, except where they have formed as druses in small 

 cavities. 



Whilst alluding to this change, I may mention, that, although 

 quartz veins have certainly resulted from intrusion of silica into 

 fissures, frequently veins of quartz in quartzite and other silicated 

 rocks are the product of secretion from the rock itself, the quartz 

 having filled up cracks produced after the rock was formed. 

 Australia abounds with examples. Here is one from N.W. 

 Australia, Brecknock Harbour. 



Mr. Sorby has examined the crystals of quartz and other 

 constituents of granite, and has discovered in them the presence 

 of visible water sometimes in cavities otherwise filled with air. 



