196 MINERALOGY AND LITHOLOGY. 



kick into a heap of sand. The rocks appear to have what was called by 

 Dolomieu, " La maladie du Granite," which he supposed to be due to 

 the action of carbonic acid issuing from a subterranean source, but the 

 true nature of the disease is discovered by microscopic examination. 

 When we look at thin sections of the granites that withstand the influ- 

 ences of time, or only wear upon the surfaces, we find that their grains 

 of feldspar are clear, and contain but few impurities or cavities. But the 

 feldspar of these Conway granites is filled with innumerable pores, and 

 with dirty matters that make it milky and opaque. Fig. i on PI. xi 

 represents a section of one of the Conway granites. The grain of biotite, 

 bent and twisted, as if acted upon by some force after it had formed, is 

 noticeable : it includes between its laminae quartz, pyrites, and magnet- 

 ite. Such grains of biotite are not uncommon. The impurity and 

 opacity of the feldspar is the chief feature, but to resolve the muddy 

 mass, a much higher magnifying power is required. Such porous impure 

 feldspars cannot withstand the influences of the weather. The water 

 enters the cleavages and pores, and soon it needs but a blow to break it 

 down. If one notice the pile of gravel that results from destruction of 

 a block of this granite, it will be seen that the cleavage surfaces of all 

 the feldspar crystals are covered with rust, which was carried into the 

 cracks from the decaying mica, but the quartz grains will be found 

 intact. Here is seen, in its simplest form, the first step of the deriva- 

 tion of soils from the disintegration of rocks. Further changes take 

 place when the feldspar is entirely decomposed and converted into kaolin, 

 for then it may be washed away and deposited in clay beds, or disposed 

 of in other ways as circumstances determine. In regions below that of 

 glacial action immense masses of such material occupy the place of the 

 original granitic rocks, and the subject has been studied in tropical re- 

 gions by Darwin, Agassiz, and others. In our parts, all products of 

 decomposition that were loosened before the glacial epoch have been 

 carried away, and only the granites that are peculiarly subject to decay 

 are deeply disintegrated in their original beds. 



The Antrim granite is 2i pseiido-porphyritic granite ; that is, it is a rock 

 in which large crystals of orthoclase are developed, not in a compact 

 ground mass, but in an already coarsely crystalline mass. Many of our 

 granites have this structure. Garnets are numerous in the Antrim gran- 



