BIGSBY PALAEOZOIC ROCKS OF NEW YORK. 365 



long exposure. The gypsum occurs in irregular isolated masses ; 

 and these are in two ranges of plaster-beds, as they are called, gene- 

 rally separated by the vermicular rock, with hopper-shaped cavities, 

 and other less characteristic masses. 



The "vermicular" rock is dark-grey or blue, and is porous or 

 cellular, like lava, which it greatly resembles. It is full of curvilinear 

 holes and variously-shaped cells, some of them having the forms which 

 are due to common salt, which has afterwards been removed in 

 solution. 



There are two masses of this rock, an upper and lower ; the 

 former extensive, the latter not so. The upper is four feet thick, 

 the other twenty. 



The lower range of gypseous masses are arched over by thin 

 calcareous layers. Just above them are, at intervals, long lines of 

 hopper-shaped cavities. They once contained salt, and are covered 

 in by a bed of the vermicular rock. These curiously-constructed 

 and arranged cavities are each composed of six hoppers, placed with 

 their apices downwards, so as to form the cube. For a more minute 

 description, see Vanuxem's Report, p. 102. 



4. The fourth or Magnesian Deposit ; so called from presenting 

 needle-formed cavities, caused by the crystallization of the sulphate 

 of magnesia. They are plentiful, as at Troopsville, Springport, &c. 



This upper mass is distinguished from those below by certain 

 narrow vertical fissures or gaps, formed by the sulphate of magnesia ; 

 by the presence of the peculiar cavities (two or three inches long) 

 first mentioned, and by the calcareous layers being more solid and 

 thick ; the softer marls, &c, having terminated with the third 

 deposit. 



The Onondaga-Salt group confirms a fact general in Europe, 

 namely, that of the association of saline springs, gypsum, and mag- 

 nesian limestone (De Verneuil, Bull. S. G. France, iv. p. 656). 



The shrinkage-cracks observed in the surface of this rock show 

 that it was, for a time, above water. They are more prominent about 

 Cayuga Lake, and in Middle New York, than further west (Hall, 

 Pal. ii. 147)*. 



The presence of so much salt and gypsum, and of some free sul- 

 phuric acid, indicates a different origin for this group from that of any 

 other in New York. At Byron, &c, in Genesee County, springs of 

 free sulphuric acid have been met with (Hall, Rep. p. 133). 



No valuable brine-springs have been found west of Lake Cayuga 

 (Hall, Rep. p. 133). 



Transition. — In New York this group came in at a period when 

 the contemporaneous rocks of England were of the Wenlock age, 

 continuously through several mineral changes, or were part of it ; 

 but in the western hemisphere this continuity of deposition was 

 interrupted for a time by the introduction of the salt-group, to be 

 covered here in its turn by Wenlock strata. 



* The Onondaga-Salt group, according to Prof. H. D. Rogers, is imperfectly 

 represented in the Appalachian Chain, south-west of the State of New York. 



2 b 2 



