FOSSIL IN CLAY SLATE. 



495 



FIG. 274. 



N.20E. 



FIG. 275. 



Quartz as a mineral is so common in all the rocks, that it is hardly worthy of notice, unless of peculiar 

 varieties, or in great quantity. In the latter respect tubercular masses of white quartz, not uncommon 

 in the clay slate, deserve to be mentioned. This quartz is nearly pure silex and is used in making 

 the purest kinds of glass. It may be found in many places, especially where the clay slate is passing into 



mica schist. A mass occurs in the clay slate, 

 or rather in the novaculite, near Bruce's quar- 

 ry, in Guilford, of such extraordinary dimen- 

 sions, and, moreover, of good quality, that it 

 deserves a description. The sketch (Fig. 274) 

 will render any further statements superfluous, 

 except to say that this great quartzose boss 

 occurs near the house of Edward F. Wilson, 

 and is eight rods long and four rods wide. 



The origin of such a huge mass of quartz is 

 a very difficult problem in geology. It is the 

 pure white quartz, not unfrequent in tubercu- 

 lar masses in the clay slate, and especially the schists of Vermont ; but it is scarcely found in any of the 

 unmetamorphosed rocks. Hence we are certain that it has undergone a change. We have already given 

 our views of the nature of the change. In all cases where we find such pure white quartz, we believe that 

 other ingredients have been abstracted from it by chemical agents, while the 

 mass was in a plastic state. But what the original rock was in this case, is a 

 more difficult question. Had the quartz formed an extensive interstratified 

 mass, we might, perhaps, suppose it originally sandstone, which was converted, 

 first into laminated quartz rock, such as occurs in Vernon and Bernardston, and 

 then by the abstraction of impurities, into the pure white massive quartz. But 

 it is a boss half as wide as it is long, having more the shape of a bed of steatite 

 or limestone. Is it possible that it may have been limestone highly silicious, or 

 which for a long time was permeated by water containing silica, by which the car- 

 bonate of lime was all abstracted ? But since this quartz is the same variety 

 which we meet with in the tubercular masses in many other parts of the clay 

 slate, but few of which could have been limestone, perhaps the more plausible 

 hypothesis is, that they are all the results of the decomposition of silicates in 

 the wet way, which seems to have been the most usual origin of quartz rock. 



FOSSIL. 



The clay slate of Guilford has yielded us at least one undoubted fossil, shown 

 on Fig. 275. It is a flattened cylinder, and by that character is distinguished 

 from the trail of an annelid, which leaves only a furrow, with all the layers of 

 the rock depressed. On the drawing a part of the stem is wanting, as it is on 

 the specimen. It looks more like the stem of a coal plant than like the fucoids 

 of the red sandrock. But we despair of being able to refer it to its true place on the scale of 

 rocks. 



Geological Position and Equivalency. 



The clay slate west of the mountains forms one of the members of the Georgia group 

 and the Hudson River group of the New York geologists, as is obvious by following the 

 ledges on their line of strike southerly into N. York. Hence its geological age is the same. 



The slate connected with the Georgia group, according to Sir William Logan, underlies 

 the Quebec group, yet is not older than the Potsdam sandstone. Professor Hall has 



Fossil in clay slate, Guilford. 



