BIGSBY PALEOZOIC ROCKS OF NEW YORK. 387 



the east of Central New York, and probably from the south-east, as 

 is evident from the thinning of the deposits, and the diminution of 

 the sandy strata, towards the west. The increase of shale westwardly 

 proves the same thing, it being lighter and more transportable. This 

 is a mere repetition of what took place in the sediments of the Hamil- 

 ton group. 



That the sea was alternately shallow and deep is proved (Hall) at 

 every step among these strata. 



The ripple-marks and diagonal lamination seen here are not met 

 with in the dark- and light-green argillaceous shales, because they 

 were laid down in deep water. These shales are very homogeneous. 



The sands were deposited in shallows ; and the occasional diagonal 

 lamination shows that the sand has been pushed over an inclined 

 plane. The shrinkage-cracks are not so large as those in the Medina 

 Sandstone. There are casts of flowing mud at Portage on the banks 

 of the River Genesee, as well as mud-furrows and striee. Many of 

 the ripple-marks have the appearance of having been caused by a 

 chopped sea, from the current opposing the wind, the ripples being 

 short, interrupted, and irregular. 



Place. — The threefold group of Portage is, in truth, the lower 

 portion of the Chemung, the next set of strata. They have been 

 separated, without very good reason, on account of the total absence 

 from the Portage of the Chemung fossils (save Clymenia complanata 

 and perhaps Atrypa reticularis), the great rarity of Invertebrates of 

 any kind, and the finer grain of its sandstone (Hall) . The Portage and 

 Chemung groups always accompany each- other, and undergo the 

 same changes. For information, therefore, as to " Place," the reader 

 is referred to the description of the last-mentioned group. 



Transition. — Hall says, very accurately, that there is no abrupt 

 mineral change in any of the rocks from the end of Tully Limestone 

 to the top of the Chemung. The Portage group rests upon Genesee 

 Slate, as at Cayuga ; and the transition is gradual. 



Position. — This rock dips very gently to the south or south-west ; 

 — or is horizontal (Hall). 



Thickness. — This averages 1000 feet ; but the rock thins off to the 

 west, with the rest of its associate strata. It is thickest on the 

 Genesee River. 



Fossils. — The fossils of this group present some important pecu- 

 liarities, induced probably by either depth or temperature. 



There are many marine plants ; but only two are distinguished 

 (Hall), namely, the Fucoidea graphica and Fucoidea verticalis (An- 

 nelid an?). 



Barren of life as is this thick mass of shale and sandstone, still it 

 yields twenty new forms. Among these is the beautiful Cyathocri- 

 nus ornatissimus, found only, and in great numbers, on a limited part 

 of the south shore of the Lake Erie, at Portland. These animals 

 seem to have all died at once, buried under the sudden accumu- 

 lation of a new mud. 



While the Chemung abounds in Brachiopoda, this group has only 



