328 



Scientific Geology. 



so that light is reflected from many of them at once when the speci- 

 men is held in a proper position, and thus a beautifully spangled ap- 

 pearance results. Not being confident as to the nature of this mine- 

 ral, I have given the rock a designation which indicates merely this 

 obvious property. These spangles are pretty uniformly diffused 

 through the mass, and their surfaces rarely coincide with the layers 

 of the slate. (Nos. 647 to 650.) 



I found this same rock in rolled masses in Newport, R. I. And 

 Col. Totten informed me that it exists in place on one of the islands 

 in Narraganset bay, — Canonicut Island, I believe. At Plainfield it is 

 sometimes divided into rhomboidal masses by oblique cross seams. 

 (No. 649.) 



7. Argillo-micaceous Slate. This exists wherever the mica slate 

 passes by gradations into clay slate ; and such places are numerous 

 in Massachusetts. It exists also, in connection with the two last va- 

 rieties, in the range of slate passing through Chesterfield, Plainfield, 

 &c. ; where the strata are perpendicular, and have a broad range of 

 decided mica slate on the east, and a similar extent of talcose slate, 

 hornblende slate, and gneiss, on the west: which position is strong 

 evidence that this rock must be one of the oldest of the primary strata. 

 It does not, however, in this case actually pass into clay slate. And 

 I believe it will always be found to consist of fine scales of mica, 

 closely compacted, so as to give it an argillaceous aspect. This rock 

 sometimes contains large beds of white quartz, which is frequently 

 fetid. (Nos. 651 to 667.) 



8. Arenaceous Mica Slate. In this variety the quartz is gray, in 

 fine sandy grains, and diffused through the whole mass, not lamellar. 

 (Nos. 668 to 712.) The mica is in fine disseminated scales; al- 

 though the plates are usually parallel to one another. The mass is 

 usually imperfectly schistose, though more regularly stratified than 

 most other varieties ; and sometimes there exists a double set of strata 

 seams. Ordinarily it is not so much contorted in its layers as the 

 older varieties ; but an intermediate variety is perhaps of all the mica 

 slates most remarkable for irregularity. The following are sketches 

 of the curvatures, in Nos. 688, 689, and 690, which are from the 

 Gorge or " Glen " in Leyden. 



