CLAY. SAND, MARL. '258 



ly been a source of confusion ; having been consi- 

 dered a primary rock, even when containing the re- 

 mains of fishes and of terrestrial plants. Schistose 

 marl occurs among the secondary strata, with the 

 shales ; and I have every where considered it as a mo- 

 dification of shale, containing calcareous matter, being 

 generally also interposed among strata of limestone. 

 If there be a distinct u marl-slate," it is also a calca- 

 reous shale, or a mixture of clay and lime,, indurated. 

 It accompanies limestone and sandstone, passing into 

 indurated clay, and into limestone. The well-known 

 Florence marble belongs to this rock. As I had oc- 

 casion to notice in the account of Organic Remains, it 

 is probable that many of these substances, when more 

 accurately examined, will prove to belong to fresh water 

 formations, and that the error has arisen from not 

 distinguishing between mere juxtaposition and alter- 

 nation. It is equally probable that many of the Italian 

 examples belong to the elevated alluvia ; as those of 

 Monte Bolca assuredly do. Under such palpable errors, 

 I have excluded this substance from its former place 

 among the primary rocks. 



The term marl is also applied to recent deposits of 

 shells,, either simple or intermixed with clay. These 

 form parts of the lacustral deposits, as already noticed; 

 but they also occur in situations where their origin is 

 much more modern. Thus they are found on shores 

 deserted by the sea, or in the actuaries of rivers ; and 

 thus also they are common in inland situations, and, 

 very often, under peat, as elsewhere described. In this 

 manner they are frequent in Scotland, indicating the 

 former places of lakes long since filled up. Their 

 geological relations are far too simple to require any 

 further detail. But J must yet point out two modes 



