THE MUSEUM. 



203, 



number of the Museum, I made a brief 

 allusion to certain beds of milky quartz 

 which are found interstratified with the 

 clay slate. In that article I had not 

 space, even had I wished to do so, to 

 enter upon a study of the quartz. Yet 

 to fully understand the clay slate form- 

 ation some description of the quartz 

 rock contained in it seems to be nec- 

 essary. 



Quartz, when pure, consists of sili- 

 con combined with o.xygen. It is 

 found in various combinations and in 

 almost every species of rock, and is 

 said to form about one-half of the 

 solid crust of the earth. Even when 

 existing nearly pure, as oxide of sili- 

 con, it is found in so many conditions 

 and forms that its study is exceedingly 

 difficult. Thus in some localities it is 

 found merely as fine sand. Again it is 

 cemented into sandstone. Often it 

 seems to be formed merely by the 

 melting of sand or sandstone. In this 

 form it is often found in the crevices 

 of other rocks, especially in granite, 

 into which it seems to have been 

 forced, when in a melted state, al- 

 though a heat sufficient to melt the 

 quartz would much more than melt the 

 granite. In other places it seems to 

 be a sedimentary rock. And still 

 again it appears to have been formed 

 by disposition — that is water contain- 

 ing quartz in solution has dissolved and 

 carried away some body already de- 

 posited and left quartz in its place. 

 This process is really petrefaction. 



These are but a few of the many 

 forms of quartz which puzzle the 

 brains of the geologist. In the clay 

 slate of Vermont it exists in still an- 

 other form. 



The previous article hinted that the 

 quartz in question was really but a 



metamorphic form of the silicous lime- 

 stone which is also interstratified with 

 the slate. Yet the hint may have 

 given rise to some erroneous impres- 

 sions. Metamorphism, when used in 

 geology, implies a change produced 

 wholly by heat. It may, however, be 

 properly used to signify any change, 

 either in the form or composition of a 

 rock. It was in this sense that I used 

 the term. 



Uuartz rock, as it exists in the slate 

 region, is of a milk-white color. It 

 has crystaline structure, is rather brit- 

 tle, and breaks into irregular frag- 

 ments. It is exceedingly hard and no 

 drill will penetrate it. There are two 

 varieties, milky and crystal, but they 

 pass into each other by insensible gra- 

 dations, and the same general descrip- 

 tions apply to both. It occurs in 

 about the same relative places that we 

 should naturally expect to find a strata 

 of limestone; but, as a general thing 

 the strata are not so thick. I know 

 of one extensive strata that is less than 

 half an inch and another that is about 

 ten feet in thickness, and between 

 these extremes there is an endless va- 

 riety. Its strike and dip correspond, 

 of course, to that of the clay slate sur- 

 rounding it. I know of no place 

 where it is found, /;/ situ, in connec- 

 tion with limestone, but limestone 

 boulders are sometimes found with a 

 thin scale of quartz upon one side. It 

 is never cemented to the slate. 



At first thought it seems incredible 

 that a rock differing so widely from 

 the original should have been formed 

 from limestone. But a little reflection 

 shows that such a result is by no 

 means improbable. This variety of 

 limestone, although it contains traces 

 of other substances, consists almost 



