76 MAGNESIAN SLATE. 



The color of this slate is usually light grey, with greenish patches. Sometimes it la 

 dark, for the same reason that the preceding slate is so, namely, (he presence of a decom- 

 posing sulphuiei. The broad layers into which die mass often readily splits, have a soft 

 pearly lustre, and a soft unctuous feel. 



The magnesias slate, however, is not uniform in its characters. It passes into a rock, 

 which at first view has been taken for mica slate ; but generally I believe it is not difficult 

 to see that it is not realh the latter rock. This is especially the case in the county of 

 Berkshire, where there are some patches which very strongly resemble the finest varieties 

 of mica slate. I certainly would not quarrel with a geologist, should he insist that some 

 portions of this rock lying towards the Hoosic mountain are really and lithologically mica 

 slate. In truth, at present it is a matter of perfect indifference with me. I do not consider, 

 :i if it should prove to be this peculiar rock lithologically, that it would affect the 

 question of its age or place in the geological systems, or that it would throw this mass 

 into the class of primary schists; for observation proves that such kinds of slate may be 

 produced in any of the earlier periods, witness the so-called talcose slate in the Old Red 

 sandstone of Cumberland in Rhode-Island. Its presence here shows conclusively that 

 such a slate may be produced when the original materials of a suitable kind are furnished. 

 Much of the ancient and primary aspect of the Rhode-Island and Mansfield beds is due 

 to the character of the surrounding rocks, which have furnished the materials; though I 

 by no meant den) the agency of a high subterranean temperature in effecting certain 

 changes. But I am not ready to admit that this is all, and that these peculiar varieties of 

 rock could have been produced independent of their original materials. 



MINERAL CONTEXTS. 



The Maguesian slate is very largely supplied with masses of milky quartz. So abundant 

 indeed is this mineral, that it occurs in boulders over a wide extent of country where the 

 slate is the principal rock. It is not in beds or veins, but it appears in irregular masses or 

 bunches. Thin seams are not of unfrequent occurence. There is often a peculiar association 

 of minerals in the Magnesian slate ; tlius, the quartz, though frequently pure milk white 

 and opake, contains bunches of chlorite intermixed with the carbonate and oxide of iron and 

 manganese. They usually appear upon the surface, like a disintegrating mass of a dark 

 color, and of a spongy texture. This rock is far less fissile than the Taconic slate, and 

 furnishes only the inferior kinds of flagging. It is less liable to decompose, and hardly 

 furnishes, when decayed, a clayey mass like the preceding slates. 



After all, but little diversity of character is found in this rock, whether we examine it 

 on the borders of the Sparry limestone, or along the eastern border of Massachusetts, and 

 ranging side by side with the Hoosic mountain. 



The minerals which appear to be peculiar to the talcose slate of the Gneiss system, as 

 Steal -tone, hornblende, etc, do not appear in this rock in New-York and Massa- 



chusetts. Of their actual appearance in this rock in one or two localities, I shall have 



