510 PHILOSOPHICAL TRAKSACTIONS. [anNO 1774. 



been extracted by Mr. Hunter. I do not find that any ichthyologist takes 

 notice of such a part belonging to fish, except Gouan, who says, that the ven- 

 tricle of some sorts resembles the gizzard of fowls, by being partly fleshy and 

 partly membranous: Gouan however does not specify the species of fish, which 

 hath such a stomach. The poke of the Gillaroo seems to perform the office of a 

 gizzard, because several small snails were found within the present specimen, 

 and I conclude, that this species of food abounds in the lakes which this variety 

 of trout frequents. 



By the best information I can procure, they are more common in Lough 

 Corryb, and the lakes of Gal way, than the other waters of Ireland : they are also 

 caught in Lough Dern, through which the Shannon runs." 



XV^. Account of the Stomach of the Gillaroo Trout. By Mr. H. Watson, p. 121. 



For a more ample dissertation on this subject, with respect to comparative 

 anatomy, the reader is referred to the paper of Mr. Hunter. 



Xyi. A Description of a Petrified Stratum, formed from the Waters of Mat- 

 loch, in Derbyshire. By Matthew Dobson, M.D. p. 124. 



During a short stay at Matlock, this summer. Dr. D. made some observations 

 on the petrifying quality of the waters, and examined a very singular stratum, 

 which has been formed in their course. This stratum he found about 500 yards 

 in length; in several places near lOO yards in breadth; and where thickest from 

 3 to 4 yards in depth. The manner in which this body of stone has been pro- 

 duced is easily ascertained. Within the memory of some persons now living, 

 the waters of Matlock were not appropriated to the purposes either of bathing or 

 drinking. They issued from near the bottom of the hill which lies to the west 

 immediately behind the present houses, and ran at random down a declivity of 

 about 100 yards, to the river Derwent. In their course they formed large petri- 

 fied masses, intermingled with great quantities of petrified moss, nuts, leaves, 

 acorns, pieces of wood, and even trunks of trees. 



The waters were thus constantly raising obstacles to their own progress, and 

 were frequently therefore forced into new channels; so as by degrees to be ex- 

 tended over a surface of at least 300 yards in length. And by being repeatedly 

 returned into the same channels, a stratum of considerable thickness has been 

 formed. On examining this stratum, some parts are discovered to be extremely 

 hard, and others so soft as easily to be cut. The soft parts however, on exposure 

 to the air, become as hard as flint; and on being struck sound like metal. The 

 reason of this difference in the hardness of different parts, appears to be this: as 

 the waters frequently changed their channels, and repeatedly likewise returned 

 again to the same channels, if, in the intervals, there were any parts considerably 



