322 G. p. MERRILL — DISINTEGRATION OF GRANITIC ROCKS. 



physical and chemical change'^ which have taken place in this trans- 

 formation and incidentally to discover the causes thereof. 



Description of Locality. 



In the accompanying illustration is shown a very typical exposure 

 as it ma}^ be seen today by the roadside on Broad branch, an affluent 

 of Rock creek, nearly a mile north of Pierce's mill. The height of the 

 bluff as here shown is not more than 18 feet. The roots shown in the 

 upper part are from plants and shrubs, as well as from trees — both pines 

 and various forms of hardwood growth, such as cover the hill — and which 

 have here been exposed through the removal of a part of the rock in the 

 work of building the roadway. As is seen in the plate, the rock is divided 

 by three principal sets of joints, one of which, running in a direction 

 nearly north and south and dipping toward the west, gives the flat sur- 

 faces facing the observer. A second series cuts across these joints nearly 

 at right angles — that is, east and west — while a third series, running north- 

 west and southeast, cuts both the other series obliquely and dips into the 

 hill at an angle of about 35 degrees from the vertical. Several minor 

 series cut these at various angles, but are not sufficiently evident in the 

 view to need mention. 



These joints have afforded passageways for the meteoric waters which 

 have been largely instrumental in bringing about the disintegration. 

 The rock in its fresh condition is a strongly foliated micaceous granite,* 

 showing to the unaided eye a finely granular aggregate of quartz and 

 feldspars arranged in imperfect lenticular masses from 2 to 5 millimeters 

 in diameter, about and through which are distributed abundant folia of 

 black mica. In the thin-section the structure is seen to be cataclastic. 

 Quartz and black mica are the most prominent constituents, though there 

 are abundant feldspars of both potash and soda-lime varieties, which, 

 owing to their limpidity, can by the unaided eye scarcely be distinguished 

 from the quartz. The potash-feldspar has in part a microcline structure. 

 Aside from these minerals a primary epidote, in small granules and at 

 times quite perfectly outlined crystals, is a strikingly abundant con- 

 stituent. Small apatites, a few flakes of a white mica (sericite), and 

 widely scattering black tourmalines and iron ores complete the list of 

 recognizable minerals. 



The outcrops from which the samples for the analyses to which I wish 

 to first call attention were selected are shown in the plate. At the very 

 bottom the rock is hard, fresh and compact, without trace of decomposi- 



*It is presumably scarcely necessary to state that the writer's views of 33 years ago regarding 

 the possible sedimentary nature of many of the District rocks have undergone a radical change. 



