tf FOSSIL SHELLS. 



in diameter. But as these did not consist of firm stone, but 

 were formed of a kind of terra lapidosa, or hardened clay, as 

 soon as they wei e exposed to the rains and frost, they mouldered 

 away. These seemed as if they were a very recent production. 

 In the chalk-pit, at the north-west end of the Hanger, large 

 nautili are sometimes observed.* 



In the very thickest strata of our freestone, and at consider- 

 able depths, well-diggers often find large scallops, or pectines, 

 having both shells deeply striated, and ridged and furrowed 

 alternately. They are highly impregnated with, if not wholly 

 composed of, the stone of the quarry, f 



LETTER IV. 



TO THOMAS PENNANT, ESQ. 



As, in last letter, the freestone of this place has been only 

 mentioned incidentally, I shall here become more particular. 



This stone is in great request for hearth-stones, and the beds 

 of ovens ; and in lining of lime-kilns it turns to good account ; 

 for the workmen use sandy loam instead of mortar, the sand 



* Modern naturalists have constituted twenty genera of those fossil 

 shells, known by the general appellation of cornu ammonis. The con- 

 clusions which geologists have come to regarding them, are these: 1st, 

 That they are first found in the formation called the lias, and appear in 

 most of the succeeding strata, but seem to have become extinct in the 

 ocean which deposited the hard chalk. The division here alluded to, is 

 what has been named the ammonacea by Lamark, which are shells with 

 a sinuous septa, lobed and cut at the margin, meeting together upon the 

 inner wall of the shell, and articulated by jagged sutures. 2d, The 

 orthocerata appear in the early strata, and are continued upwards to the 

 soft chalk stratum, after which they are not seen. These shells are 

 straight, or nearly so, and not spiral. 3d, The oval ammonitaz are not 

 known in the early strata, but m the hard chalk only, and are not seen 

 afterwards, as if they had been created at a comparatively late period, and 

 had been soon suffered to become extinct. The shells alluded to by our 

 author, which mouldered away, had been the impressions only of these 

 cornua ammonis. ED. 



f In Corncockle Moor, Dumfries-shire, there is a sandstone quarry, oil 

 the slabs of which are distinctly imprinted the tracks of the foot marks of 

 animals. These were discovered in the year 1812. They differ in size 

 from that of a hare's paw to the hoof of a pony. On a slab, which forms 

 part of the wall of a summer-house, in Dr Duncan's garden, at the Manse 

 of Ruthwell, there are twenty-four impressions, twelve of the right, and 

 as many of the left foot. Professor Buckland considers that the animals 

 must have been crocodiles or tortoises. En. 



